Mine are:
Tweetie 2
Facebook
ego
remote (and keynote remote)
and doodle jump (my current game right now for a little fun, every few weeks i'll pick up a new game to pass idle time this one is pretty fun)
Midomi SUCKS, I can't believe it's one of your top 5. I used it along with Shazam for the longest time, convinced that they would both be useful at some point when the other failed. I was wrong. Shazam recognizes every track midomi does, but when midomi failed because of background noise or simply did not recognize a track, Shazam almost always came through. Only my nerdiest indie obscurities go unrecognized- Shazam is better in every way. With that out of the way my 5 indispensibles are:
2Do
Shazam
NowPlaying
GuitarToolkit
Sportacular
...and ok, I know it's the 6th but AirSharing has to be included too