Trainyard is my favorite puzzle game I have found bar none. It is one of those easy to grasp, hard to master games that gives a real feeling of accomplishment with each puzzle completed.
Doodle God/Devil is fantastic. Not much replay value to me, but it can take a good while to play through them.
Galaxy on Fire II. I've logged so much time on that game it's unreal. Plus, I love that you can use OpenFeint to save your game on the iPhone and load up the same game on the iPad.
I always go back and play Tiki Totems.. And "chain link". Very fun and addictive games
Oh, and shamless plug. Santa's Delivery . Now that it has gone live with the leaderboards being filled with scores I find my self up at night playing to keep the highest score
Plants vs. Zombies. I also enjoy Doodle Jump, Flight Control, Mega Jump, Cut the Rope, and Bejewled. I don't get the fun in Angry Birds. It's just frustrating to me.