01) Captain America: The Winter Soldier
02) The Avengers
03) Iron Man
04) Captain America: The First Avenger
05) Thor
06) Thor: The Dark World
07) Iron Man 2
08) Incredible Hulk
09) Iron Man 3
I can't rank Guardians yet, as I've only seen it one time and I want to see it again first. I'm guessing it winds up somewhere between #3-5.
The First Avenger and Thor are always tough for me to choose between. Thor used to always be above TFA for me, but now when I watch TFA I have TWS in my mind, and those two movies just flow together so well. There's very little doubt to me that Cap's series is going to be the reigning champion for me, out of all the Avengers' solo outings.
Favorite wise, I probably like both Thor and TFA better than Iron Man, to be honest (due to more attachment to the characters). But Iron Man is the superior movie, which is why I ranked it 3rd.