MVP: Iron Man, Spider-Man, and Strange for me. These three had great chemistry, got the two best action scenes in the film (the beginning set piece in New York and then the battle on Titan) and Spidey's death hit me the hardest. Plus, I think Stark in particular had the best character arc of the heroes (with the exception of Thor). He was forced to confront Thanos, his literal worst fear, and Downey sold the sheer panic Stark was feeling. Strange, meanwhile, was clearly the brains, and I think his actions will ensure that half the universe is saved in the long run. Spider-Man, meanwhile, was the straight forward good guy, just doing what he could to save lives (him frantically saving all of the Guardians was a great little moment). Plus, the fact that these three are my favorites from the comics probably doesn't hurt, either.
MDP: Captain America. He was just..there. No inspiring speeches, no really cool moments (aside from holding off Thanos at the end). He just showed up and acted vaguely like how you would expect Cap to act. He was still good, but he didn't really get anything exceptional to do. This was disappointing for me, especially after seeing how well the Russos have handled him before.
MIP: Thor. While I have always liked Hemsworth in the role, and I think he had several great moments in Avengers and AoU, I don't care for any of his solo movies (no, not even Ragnarok, though I know I am in the minority there). Here, he was great, portrayed as a grieving man with nothing less to lose, but he still had the compassion and heroism that make him likable. He got the character arc he should have gotten in Ragnarok, and I loved it (even if the sequence of him forging Stormbreaker dragged a bit for me). His arrival at the end was one of the highlights of the film.