Here are a few of Xander's favorites:
Elmo Loves ABCs (for iPad only) - this is his newest favorite
Scout's ABCs - see my earlier post about this one
My First Puzzles
Itsy Bitsy by Duck Duck Moose
The Wheels on the Bus also by Duck Duck Moose
I Hear Ewe (he doesn't care for this one much anymore, but this was the first app I got for him - it's free.)
But probably more than anything (except possibly Elmo) he loves to play ANGRY BIRDS! Yes! That is one of his favorites and he can play it for (probably) hours (if I let him). He plays the original Angry Birds, Angry Birds Rio, Angry Birds seasons, even the free versions! He loves it!