if i had to tier certain classes right now, i'd say that:
S tier
vein - very high dps, good healing, good mobility. beats up melee guys pretty easily, can chase down anyone on a horizontal plane and still hit them.
sorceress - even with the teleport nerf, the extra 20 hp helps a lot. can run gambit on anyone on any of the tdm maps.
dante - hardest class to play, but probably the most rewarding if you know what you're doing. has the tools to deal with any matchup.
any vault boy with high hp and gauss rifle - self explanatory
edit: I want to make a small-ish tdm map for 2-6 players (1v1 to 3v3). Any ideas on what the theme should be?