I used to Dragonvale, but after the game ****ed me over and didn't allow me to get one of the gem stone dragons and I tried EVERYDAY ALL DAY, for the whole month, i gave up and now i don't play it anymore, and I really miss it, but :/
Tweetbot probably. The whole Tapbot design scheme is so nice and fresh to me. I haven't really found a twitter app from anywhere that compares to it. It's feature packed too.
For games: World of Goo on the iPad is probably the best game I've ever played on an iOS device. It's kinda easy but the soundtrack + everything gives me so much life. It's like a fully produced console game. I know it was on Macs/PCs first but it really needs a touch screen for it to really work. Rolando 2 + 1 are also really good.
Cut the Rope is a great series and really fun to beat (Experiments, Seasons, original).
The Kayak app is helpful for looking up flights + renting cars on the go.
I use TV guide almost every other day or so to find channels and when certain shows come on. It's really helpful and the new update is >.
The Trailers app is nice for people who like to see what movies are coming soon easily.
I use the United app a lot since I travel with them mostly. Mobile boarding passes are really nice + I'm excited that they'll all be in Passbook with gift cards and everything else in iOS 6. Really helpful.