Galcon, Fastlane, and Chess With Friends are what I play the most, and I've downloaded a lot of games. Fieldrunners and Aurora: The Arena are also great.
I've been too busy to get into Brothers In Arms, Hero Of Sparta, Warfare Incorporated, Legend Of Feanor, or Vay. They all from the little I've played are not bad for being on an iPhone platform (well Vay is just oldschool RPG nostalgia).
It's kind of hard to recommend a chess app when we don't know the measure you are using for best. In terms of learning new moves? Online realtime play? Games that don't require you sit one time for a long time to finish? Chess With Friends is my favorite, cause I don't usually have time to sit down and finish a game from start to finish, followed by Deep Green, it has a great interface, nice look and animations, and it usually kicks my ass on the easiest setting (I'm rated at 1350-1400 on Yahoo chess).