I always liked the low powered and no-powered characters who didn't get much exposure (Red Wolf is a good example), but I could hardly name someone like Gemini or Razorback, both of whom had very few appearances, as a favorite because they are too obscure to be recognized by most readers. Therefore, I'll go with Hawkeye, who should be well enough known (he has appeared in two short lived cartoon series, so even the non-readers might know who he is).
Hawkeye has been around for years, and was first shown as a pawn of a villainess and a foe of Iron Man. Hawkeye then became one of the first replacement Avengers, which resulted in a long association with that team. Hawkeye is one of the few Marvel characters who has actually grown as an individual, evolving from being a rude and self-centered loudmouth into an effective leader, who can be counted on take care of his team members. Various writers have done horrible things to Hawkeye over the years, like kill his wife and even apparently kill him, but he always shows up again and does his best. He has no actual powers, but holds his own in very strong company and can be counted on to watch other peoples' backs.