Content Warning: This article contains references to suicide
The story of the Doppelgängers formed the foundation for the plot of The Vampire Diaries. From the very beginning, Stefan and Damon are shown as being pulled towards Elena Gilbert because she is a doppelgänger of the 500-year old vampire, Katherine Pierce.
In the show, there are two doppelgänger "lines" - one being the Salvatore line and the other the Petrova. While Stefan and Elena were the first two dopplegängers the viewers were introduced to, as the show went on, the writers did begin to include a lot more of them, with some of the characters affecting the series in huge ways (especially when some turned out to be the main antagonist for one or more seasons). But which one proved to be the worst of all? While some of the doppelgängers didn't get arcs that were as in-depth as the others, they certainly made a huge impression on the viewers by committing to horrific deeds overall.
Tom Avery
Tom Avery was a doppelgänger of Stefan and Silas, appearing in the season 5, "While You Were Sleeping." In the episode, as Tom tells Caroline about his life under her compulsion, he is discovered to be an extremely good human being.
Tom is a paramedic who saves lives regularly and his colleague called him the best paramedic he has ever met. Judging from his minor appearance in the show, he seemed like an extremely and kind person, who can never hurt someone knowingly. Sadly, he was killed by Enzo but it wasn't surprising because the fates of doppelgängers usually end up in pain or death. Strangely, Stefan and Damon weren't too affected by the loss since they didn't even know the doppelgänger existed until that (which was something that confused TVD fans).
Tatia
Tatia lived in the late 10th/early 11th century, with whom both brothers, Klaus and Elijah Mikaelson, fell in love with. Her blood was used by their mother, Esther Mikaelson, to turn all her children into vampires.
Tatia wasn't necessarily an evil character. She was a headstrong widow who wanted to fall in love again. However, when Elijah became a vampire, she was incredibly rude and called him a monster (even though she once claimed to love him). This doesn't justify her death at Elijah's hands, but it shows that she never really loved him.
Amara
Amara was the first immortal woman in the world of The Vampire Diaries and the originator of Petrova doppelgängers. She was cursed by a powerful witch, Qetsiyah, after the witch learned about her love affair with Silas (who had been engaged to Qetsiyah).
Amara was overall a good person and her only fault was betraying Qetsiyah and falling in love with her husband-to-be. When she wakes up from her curse, she is deeply pained by feeling every supernatural death over 2,000 years and kills herself, signifying that she was more compassionate and humane than Silas.
Stefan
Stefan Salvatore is Silas' doppelgänger and one of the protagonists of The Vampire Diaries. He was turned into a vampire by his 'lover' at that time, Katerina Petrova, and who is also the doppelgänger of Elena.
Stefan has been a morally ambiguous character throughout the show. Being a vampire, he has committed some of the most heinous crimes, but he constantly tries to control his bloodlust. Most times, Stefan is capable of being good person, but there are periods when he gives in to his beastly nature and becomes incredibly dark (like when he became the Ripper).
Elena
Elena Gilbert is the former female protagonist of the show. Being the doppelgänger of Katherine, she attracts the Salvatore brothers to Mystic Falls. Elena was an innately good character at the beginning of the show but her morality gets hazy when she becomes a vampire at the end of season 3.
Even though she never wanted to be a vampire, she went on a dark path after becoming one. She killed many people, including an innocent waitress just to prove to Damon and Stefan how much she enjoyed turning her humanity off (among other things). She even tried to kill her best friends, Bonnie and Caroline, multiple times. This shows that there is an innate darkness inside her that could prove to be dangerous if she spiralled out of control.
Katherine
Katherine or Katerina Petrova's introduction at the end of season 1 kicked off the doppelgänger storyline in The Vampire Diaries and she was the main antagonist for many seasons. As evidenced by her storylines, Katherine has led a troubled life and has suffered a lot, especially at the hands of Klaus.
This made her into a cold-hearted villain, who loved to manipulate people. She constantly manipulated Stefan and Damon and pit them against each other. She also possessed Elena's body, got the Salvatore brothers killed, forced Tyler to turn, killed Caroline, fed Jeremy to Silas, and also killed many innocents throughout her life.
Silas
Silas is the first immortal man in the show's universe and is also one of the most powerful TVD villains. He was a witch and engaged to Qetsiyah, but he betrayed her and cheated on her with Amara. When he came back to life after thousands of years, he locks Stefan in a safe and threw him into the water, where Stefan went through a repeated cycle of drowning, dying, and coming alive for months.
He has done many other horrible things (like killing Jeremy), draining the cure off of Katherine that sped up her aging process and natural death, and using Bonnie to kill witches. This is why Silas is the most evil among all the doppelgängers, and one of the most evil characters in the show.