I have a HM slave vaporion for black two
it knows Surf, Dive, and waterfall XD
It has a **** ton of health + water absorb so it makes a pretty good wall. And against fire and/or ground types it doesn't do too terribly.
I usually keep the HM's on just in case but I rarely use them besides the required puzzles.
Which I wish there were more of, just not 'required' to beat the game.
On a side note, I have a team of all eeveelutions and they're all level 100 =-=