My top 5 are Chorus, Appolicious, FastMall, Scramble and Shazam.
Chrorus is a great way on the iphone to find other apps you may not have known about, same with appolicious.
FastMall is an incredible new platform experience that lets a user shake their iphone for the nearest restroom at many shopping malls along with turn by turn guidance inside the mall, gps remember where you parked, shopping lists, full community and crowdsourcing along with getting deals.
Scramble is tons of fun to play vs other word maniacs like me online. (Id put scrabble here too).
Shazam is just incredible technology, highly recommend.
Happy New Year 2010 everyone!