My personal list:
1. Captain America/Iron Man
2. Iron Man/Captain America
I lean Cap but sometimes I'm in a Tony mood. These are always my top 2 though.
3. Black Widow
4. Spider-Man
5. Daredevil
6. Ant-Man
7. Thor
8. Jessica Jones
9. Black Panther
10. Punisher
11. Luke Cage
12. Guardians of the Galaxy
13. Falcon
14. Hulk
15. Peggy Carter
16. War Machine
17. Scarlet Witch
18. Vision
19. Hawkeye
20. Quicksilver
Haven't seen Doctor Strange (yet) or Shield. Some like Panther have a chance to climb. I have a feeling his solo movie is gonna shoot him up for me. Marvel does a really good job with its lead characters tbh. A lot of hate for the villains (and most are pretty weak) but the actual heroes are very likable and well characterized. I feel like Marvel doesn't get enough credit for that. 1-17 are all characters I like a lot. Vision has been pretty bland to me so far but he was much better in Civil War then Ultron. Love comic book Hawkeye but still not feeling him in the movies lol and I keep forgetting that Pietro was even in the MCU