Hard to say.
I kinda like Grunt and his commando sweater wearing ugly mug. But the 1991 verison didn't cut it. To me the basic infantry man is the heart of the team, not the loose cannons, bad arses and grand standers. To that end, Crazylegs, Dusty and Footloose are cool, too.
Also, the niche specialists, Barbecue, Airtight...some of those are nice.
Low Light has a nice anti-hero air to him. But then, along comes the 1991 mold and it messed with my view of the character. Still, he hasn't been totally fanwanked to death the way Beach-Head, Flint and Snake-Eyes (well, he was officially beaten into the ground) have.
Honestly, I've often viewed the female characters as just window dressing. That's because they all have to be attached to a male joe. To that end, Cover-Girl is probably my fav of the lot because she's a "free agent'

and not an undercover/intell whatever type (no matter what Devil's Due thought).
But to get old school, 1982 crew aside, the ones I played with most in the day were probably Airtight, Recondo, Beach-Head, Lowlight, Duke, Crazylegs, Dusty, Crankcase, Dialtone and Roadblock V2 (i didn't have v1). Of course that based on what I had so...