Of course this will always be subjective, but mine are (in no particular order):

VNC viewer - Great for accessing my PC on the go.
MPlayer - Mainly for audio playlists.
VGBA - I'm addicted to Scrabble (among other games) and don't have to carry my Game Boy Advance anymore.
Tram Tracker - I live in Melbourne and don't drive, so it's a godsend.
Flashlight - Boring to add to the list, but I use it so often and missed it on my last phone.