I demolish everyone unless I'm facing:
Big Daddy (his darn Down-Triangle move)
Raiden+Dante in 2v2 - If people get these two and have a plan for using them together its hard to get past all the crazy combos
In 1v1 stock matches, there are a few impossible (I'm using the term loosley) match-ups I've noticed. I play as Ratchet, so I'll give a few examples with just him:
Raiden vs Ratchet - Raidens combos and range and ability to glide instantly across the screen make this an impossible matchup. I am a level 327 ratchet and have a very good control and understanding with the character, and was destroyed (3-1)by a level 27 raiden. The map was bioshock infinity. I tried many different strats in this match-up. Typically against a combo heavy character I would keep my distance, use my side square wrench attack or pistol if they get close, but raiden can glide across the map, and use air attacks that would destroy my telsa spikes. Very frustrating.
Heinchi (the fighter guy) vs Ratchet - I really have no idea how Heinchi can beat Ratchet in this matchup. Ratchet has such solid ranged attacks, as well as using his telsa spikes to prevent heinchi from reaching him. Easiest 1v1 match-up I've ever experianced, haha. I find his bear is pretty easy to evade as well depending on the map in a 1v1 situation