In no particular order:
Tom Clancy's H.A.W.X.
I have not tried many games, but of the ones I have tried those five have lasted on my phone. I did not know this forum existed though, and found two more games to be interesting I will be trying now (Mushroom Wars and Castle Warriors 3D).
Gameloft has some other cool titles; and their LIVE thing, along with online play and achievements adds to replayability with many of the titles too. I've always been a fan of them since I first played Rayman and Siberian Strike back on my Palm m505.
I really want some more sports games though...Madden 11 in particular.
Also, a "real" RTS game would be awesome. Spiff Code has been thinking abou porting their game, Warfare Incorperated to the Android market as well. I remember testing it back in the day on Palm devices, truely a huge advancement in RTS games for mobile devices.