Nip-It is pretty, and has a cool gimmick with the gator button, but I don't find it to be one of the better games to play.
Top Score is in my top few EMs, and I think is in the top 5 on Pinside's top 100 list. I would definitely buy that over damn near anything else.
Wedgeheads are nice, but prices seem to be inflated; I tend to keep a few multiplayer EMs and one wedgehead set up.
There was also mention of a Grand Prix; I like that one a lot as well - probably my favorite Williams EM.