For replay value I love Words with Friends. Been playing it for months. There is always someone available on the net to have a game with. A neat feature is you can have several games going at once.
I also like Scramble but they don't have an iPad version so the graphics don't look great.
I'm not a gamer at all, but Plants vs. Zombies, Osmos, and Inifinity Blade are my iPad games. PvZ doesn't have much replay value, in my opinion. Osmos is perfect for unwinding. Terrific electronic soundtrack. Inifinity Blade is repetitive, but entertaining nonetheless.
As a PopCap game lover, I have Peggle for the iPhone and I think it's great.