What is the most complicated relationship in the entire MCU? That's a difficult question. From Nebula's hate for her father to Tony's family issues, the MCU has plenty of complex bonds between its various characters. But one of the relationships that had the strongest impact on the storyline exists between Tony Stark and Steve Rogers.
The two superheroes first met during the Avengers (2012) but they already knew of each other's existence. They didn't immediately get on well, clashed on more occasions than one. However, in time, they became friends. Despite that, there were some instances when Tony and Steve didn't act like friends should.
Weren't: They Didn't Listen To Each Other
The start of the conflict between Tony and Steve was caused by the fact that they didn't listen to each other. Instead of letting the other one explain his attitude, why he believed he was doing the right thing, they clashed about the Accords from the start.
In the end, both Tony and Steve were convinced their actions were justified, and neither gave the other the opportunity to explain himself. Friends should be able to communicate in a better way.
Were: Steve Missed Tony
Sometimes the briefest of moments in the MCU were the ones that proved Tony and Steve were friends. At the end of Avengers: Age of Ultron, despite their arguments throughout the film, Tony and Steve say goodbye in a peaceful fashion. After Thor departs and it's just the two of them, Steve accompanies Tony to his car and confesses: "I will miss you, Tony."
Steve's not the type of person to make up something like this so it's safe to assume he meant it.
Weren't: Steve Didn't Tell Tony About His Parents
Still, despite their different attitude toward the Accords, Steve and Tony could have solved their differences in a more peaceful way. But any chance at immediate reconciliation between them ended when Tony learned that Steve lied to him about the death of his parents.
Or, more precisely, didn't share the truth. Steve knew that brainwashed Bucky had killed Tony's parents, but because he wanted to protect Bucky, he kept the information from Tony which seriously damaged their friendship.
Were: But He Wanted To Be There Nonetheless
In one of the most hilarious scenes of Captain America: Civil War (2016), Stan Lee, who plays a postman this time, shows up and gives Tony a package from Steve. It turns out that despite everything that happened between them, Steve still wants to stay in touch with Tony in case he needs it.
It's a nice gesture from him, and the films even later return to it. Tony considers calling Steve in Avengers: Infinity War (2018) but chooses not to do it in the end.
Were: Tony Acknowledged It
Just a short time before Tony ends up fighting both Steve and Bucky, he actually acknowledges the friendship between them - which is as good an indicator as any that they indeed used to be friends.
When Steve explains he has to protect Bucky because, quote: "I’m sorry, Tony. You know I wouldn’t do this if I had any other choice, but he’s my friend.", Tony responds with three simple words: "So was I." That's why Steve's betrayal stings him even more since he considered him his friend.
Weren't: They Said Some Cruel Words To Each Other
It's true that tensions were high that day on the Helicarrier - courtesy of Loki's scepter that brought disagreement among the newly-established Avengers. But it's difficult to overlook that Steve and Tony exchanged some hurtful words that day. Steve pointed out that Tony wasn't willing to sacrifice himself for others, and Tony called Steve a lab rat.
Luckily, they reconciled soon after and eventually proved each other wrong.
Were: But They Also Joked Together
There was a lot of drama going on between Tony and Steve - after all, they did have a lot to deal with, from aliens to gods. But there were also some heartfelt, relaxed moments where they simply joked together as friends do.
One example of this was when Tony told Steve not to steal the chopped wood from his pile - even though Steve's pile was twice bigger than Tony's. Tony also made friendly fun of Steve because of his tendency to correct people's crude expressions by saying "Language!"
Were: Tony Gave Steve His Shield Back
Considering the fact that Tony took the shield from him in Captain America: Civil War, and considering everything else that happened between them, it would make sense if Tony held onto the shield.
But instead, he acknowledges that it belongs to Steve after all and gives it back to him in Avengers: Endgame (2019). This moment also works as Tony handing an olive branch of peace to Tony since he says he no longer wants to hold to the resentment between them.
Were: They Trusted Each Other (And Went Back In Time)
Even though they didn't part on friendly terms a few years back, Steve and Tony still reunite in 2023 and decide to travel back in time together, back to the 1970s to get the Space Stone. Before they realize the journey, Tony asks Steve: "Do you trust me?", to which Steve replies: "I do."
It's a brief moment in the 3-hour film but shows that the duo has moved past their differences and they're willing to work together as friends and teammates once more.
Were: Steve (Almost) Kissed Tony
Finally, there's the matter of the almost-kiss between Steve and Tony. The scene never made it into Avengers: Age of Ultron but at least ended up in bloopers.
If Steve and Tony weren't true friends, it's doubtful Tony would let Steve get so close to him - or that Steve would go for a kiss in the first place. It's a sign of true friendship.