My absolute favorite is a toss up between black suit Spiderwoman and Classic Storm.
As far as the most valuable I'm not sure, but I have the most of the ones that are either harder to find or more expensive in my opinion: Nightcralwer, black suit Spiderwoman, Iron Fist, Nick Fury. Ben Reilley and Hobgoblin 2-pack, and Dark Phoenix variant from the X-3 2-pack.
Damn Super Skrull and Stealth Iron Man elude me.......
And how could I forget my other favorite, Crippler's super bad ass custom 3 inch Giant Man that thing is so pimp