I am all about Scarlett ... so has to be Black Widow.
Umm, I don't know many, so after a quick Google... Iron Man stands out for me.
The ever underrated Ant-Man was a close second purely because of his name.
If you mean the UK version, Purdey from the New Avengers (Joanna Lumley)
Honor Blackman (but still second to Diana Rigg)
Ultimately, I like Thor and not just because I've always been fond of Norse mythology. The other Avengers "superpowers" are all based on the artifice of technology. Captain America's serum, Banner's accidental and unintended exposure to artificially generated gamma radiation and "Iron Man" Stark accurately describes himself as just "a man in a can." Thor on the other hand draws his powers directly from the universe. His powers are not based on the crutch of technology, but are mystical and organic. And what great powers they are. If Iron man gets knocked out, he has to change suits. When Thor needs a boost, he strikes himself with lightning and that helped create one of the coolest scenes ever in movies, as well as one of the most infuriatingly brief ones. I also like his sense of noblesse oblige, something he shares with Captain America who, especially after watching Winter Soldier, runs a very close second to Thor as my favorite Avenger.