It's hard to believe that Agents of S.H.I.E.L.D. is now in its fifth season. The series has gone from strength to strength, and is now Marvel's highest-rated show on Rotten Tomatoes. It's about to celebrate its 100th episode, a milestone that promises to feature faces from the past even as it continues the current arc.Related: If Agents Of S.H.I.E.L.D. Season 6 Happens, What Would It Be About?Coulson and his team have come a long way since they first assembled in Season 1. S.H.I.E.L.D. itself has changed shape countless times; it was brought down by Hydra, but resurrected by Coulson himself. The show's future is currently uncertain, although ABC President Channing Dungey is "cautiously optimistic" for Season 6. Whatever the future may hold, though, now is the perfect time to cast an eye back to the past and celebrate S.H.I.E.L.D.'s best moments.This Page: May and the Asgardian Staff, Skye's Terrigenesis, and Ward's Turn

10. Agent May Wields The Asgardian Staff (Season 1)

Season 1 tended to tie into the movies a little too much. Still, one of the most effective tie-ins was 'The Well'. It saw Coulson and his team hunt down an Asgardian staff weapon. Anyone who wielded this staff became a berserker, and it was being used to cause riots in Scandinavia. Holding a third of the staff left Ward tortured by visions of his past; impressively, although the episode teased Ward's demons, it somehow still avoided revealing his greatest secrets.

Towards the end of the episode, the story took a twist. May took up not a third of the staff, but the whole weapon. Incredibly, she proved able to wield it without effect. Later, Ward asked May how she'd resisted the pain of what she'd seen; in a haunting character moment, May explained that her regrets were always with her. She faces them every day when she wakes up.

9. Terrigenesis (Season 2)

When Stan Lee came up with the idea of mutants, it was really because he'd become tired of writing origin stories. With mutants in play, he no longer needed to imagine radioactive spiders, super-soldier serums, or Cosmic Ray bombardment; superhuman powers were just triggered by a fluke of genetics. For Agents of S.H.I.E.L.D., the Inhuman gene would come to serve a similar function.

That all started with "What They Become," the Season 2 episode that saw Daisy undergo Terrigenesis. This was essentially a death-and-resurrection scene for the character. Skye, the hacker who longed to be a S.H.I.E.L.D. agent, died; she was replaced by Daisy Johnson, a.k.a. Quake, who would effectively become S.H.I.E.L.D.'s lead superhero.

RELATED: How The Inhumans Finale Could Effect Agents of S.H.I.E.L.D.

8. Grant Ward's Betrayal (Season 1)

Fans had spent the first half-season complaining that Brett Dalton's Grant Ward was simply too good to be true. It wasn't unusual to see reviews argue that no secret agent and assassin could ever be so nice a person. Ward just seemed to break fans' ability to suspend their disbelief. Then, in a dramatic twist, Agents of S.H.I.E.L.D. revealed that Ward had been an agent of Hydra all along. The Ward fans had seen in the first half of Season 1 was nothing more than a mask. He really had been too good to be true; that was precisely the point.

Even more impressively, S.H.I.E.L.D. revealed this in a last-second twist. Hydra seemed to have been defeated, and fans were beginning to wonder what would come next. Then, in the final sequence of 'Turn, Turn, Turn', Ward shot down some S.H.I.E.L.D. agents, murdered Victoria Hand, and rescued key Hydra leader John Garrett. It was an unforgettable moment and would reshape the whole series.

7. Jemma Simmons, Agent Of Hydra (Season 2)

Season 2 kicked off with a mystery: where was Jemma Simmons? Fans briefly thought she was still with S.H.I.E.L.D., but that turned out to be a hallucination, conjured up by Fitz's damaged brain. Dialogue revealed that Simmons had left, unable to handle Fitz's brain damage. Finally, the teaser for 'Making Friends and Influencing People' began to answer those questions. It was an effective sequence, with Simmons seeming to have embraced civilian life. The music choice was inspired, with Belle & Sebastian's 'God Help The Girl' suggested she was simply trying to get over Fitz. Then, the camera panned to the wall of Simmons's workplace and revealed she was working for Hydra.

This was easily one of S.H.I.E.L.D.'s most artistic scenes. It may not have had a long-term impact on the series, but it was handled in a remarkably creative way.

RELATED: Agents of SHIELD Star Calls Potential Movie Crossover Tricky'

6. The Death Of Grant Ward (Season 3)

It took three seasons for Grant Ward's story to come to an end. Ward had made so many enemies over the years; he'd seduced May, dumped Fitz and Simmons in the ocean, and messed with Daisy's head. But when he killed Coulson's lover, he earned the enmity of S.H.I.E.L.D.'s director. What followed was an intensely personal confrontation on the cursed planet of Maveth.

The sequence was a powerful one, with Coulson using his cyborg hand to literally crush Ward's chest. It was a fitting end to Ward, but it left fans with an uncomfortable taste in their mouths. Coulson had just crossed a line; he hadn't killed for peace or justice, but out of a sense of vengeance and betrayal. In a fantastic twist, though, this action would return to haunt S.H.I.E.L.D.; Ward's body was possessed by the Inhuman parasite Hive. Coulson's act of revenge actually gave Hive a way back to Earth, and almost led to humanity's destruction.

5. Enter Mockingbird (Season 2)

Fans were thrilled when Marvel announced that Adrianne Palicki would be joining the cast of S.H.I.E.L.D.. Not only was Palicki rightly viewed as a phenomenal actress, she was playing Bobbi Morse, better known as the superhero Mockingbird.

What nobody had expected, though, was for Morse to be introduced as Hydra's security chief. It wasn't hard to guess she was an undercover operative, and that was confirmed when she rescued Simmons from a bunch of Hydra agents. This was one of Marvel's first "Hallway Scenes," showing Morse cutting a swathe through Hydra's rank and file. It was vaguely reminiscent of Black Widow's stunts in Iron Man 2, setting Morse a cut above the rest of the cast in terms of the action.

RELATED: Adrianne Palicki Wants Mockingbird / Black Widow Crossover

4. Quake Goes Rogue (Season 3)

In an interview with Entertainment Weekly, Chloe Bennet noted that Season 3 was "the end of the first book of S.H.I.E.L.D." The season closed with a fascinating scene, one that teased a whole new direction for the show. It was set six months after the events of Season 3, and revealed that Quake had actually gone rogue. She was blamed for everything from bank heists to destroying bridges. Even more astounding, Coulson was back as a field-agent, and he and Mack were hunting Daisy down.

It was an effective tease, and left fans eager for Season 4. It was also one of the coolest scenes to date, with Daisy using her powers to allow limited flight for the first time. That made this sequence a power-up for the character who was becoming S.H.I.E.L.D.'s own superhero.

3. The Spy's Goodbye

By Season 3, Bobbi Morse and her ex-husband Hunter had become major characters. So much so that Marvel believed they could become the stars of their spinoff, and went so far as to write them out of Agents of S.H.I.E.L.D.. In hindsight, it was a mistake; ABC passed on Marvel's Most Wanted.

The Season 3 episode 'Parting Shot' was essentially a goodbye story for Morse and Hunter. The two agents prevented a rogue Inhuman gaining control of Russia, but they did so at the cost of their own cover. It took all Coulson's best efforts to get them out of jail, but they could never work for S.H.I.E.L.D. again. At the end of the episode, Hunter and Morse settled down in a bar to share a drink and decide what to do next. That was when someone bought them a drink - Simmons, sat at a nearby table. It turned out the whole of the S.H.I.E.L.D. team were on hand, and each quietly bought them a shot. Bobbi recognized this as "the spy's goodbye," a farewell from friends who could likely never speak to them again.

RELATED: Lance Hunter Returns To Save Fitz In Agents of S.H.I.E.L.D.

It was one of the most emotional moments in S.H.I.E.L.D. history, with each actor demonstrating all their skill in presenting the complex weave of emotions; grief, guilt, sorrow, respect, and regret, all this and more were in play. The scene was supported by beautiful music, hammering home the emotion of it all. By the end of the episode, Jemma Simmons and Bobbi Morse weren't the only ones in tears.

2. Quake Versus Ghost Rider (Season 4)

Of course, there was a lot more to Season 4 than Daisy's going rogue. Fans soon learned that Agents of S.H.I.E.L.D. was about to debut a whole new Ghost Rider, Gabriel Luna's Robbie Reyes. Expectations were high for the season premiere, 'The Ghost', which soon saw Quake and Ghost Rider go head-to-head. It's something of a superhero tradition for heroes to fight one another when they first cross paths, and S.H.I.E.L.D. was happy to oblige.

Fans were spellbound, not least because of the quality of the show's special effect. Marvel spared no expense in creating this new Ghost Rider, rendering him more effectively than even the Ghost Rider movies. That did limit Marvel a little, and they swiftly had to write Ghost Rider out due to costs; he'd ultimately return in the season finale.

1. The Death Of Hope (Season 4)

Season 4's final pod was centered around a virtual reality known as the Framework. While most of the S.H.I.E.L.D. team were glad to leave this reality, which was ruled by Hydra, Mack was the exception. He learned that, in the real world, his daughter Hope had died years ago. Even though Mack knew Hope wasn't real, he simply couldn't leave her behind.

Then, in a heartbreaking twist, Aida began to shut down the Framework. It all came to a head as Mack cradled his beloved daughter in his arms, desperately hoping not to lose her. Then, in a single split-second, she was gone. Henry Simmons played the scene perfectly, showing Mack's utter heartbreak. It was one of the most moving scenes there's ever been on S.H.I.E.L.D., and would leave his character scarred through into Season 5.

These are just some of the best scenes in Marvel's Agents of S.H.I.E.L.D. to date. The series has gone from strength to strength, and the characters have become wonderfully three-dimensional. So many of these key moments are ones that are personal and intimate; moments of grief and pain, hope and fear. Over the last 100 episodes, Coulson and his team have developed a firm fan-following. Here's hoping their story will continue on after Season 5.

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