I won't mince words here: I've seen and played a lot of iOS games, and when I first saw Casey's Contraptions back at GDC this year (which is out today for the first time on the iPad), I thought it was the most impressive iOS game I'd ever seen.
The Incredible Machine is what jumps to mind to describe it, but really, this game is more than that -- it's a full engine of physics gameplay. The idea is that you're presented with over 70 included puzzles by young Casey, and then you have to solve them with various items, which span everything from punching gloves to darts and RC cars. After that, you can also create your own levels and even share both created levels and solutions around with an incredible social backdrop. The whole app is very impressive -- the design is just perfect, there's a huge number of items to interact and build with, and the social features are really amazing and incorporated seamlessly through email and Game Center.
Seriously, if you own an iPad, Casey's Contraptions is pretty much a must-have title. There are content and feature updates on the way as well (and if you don't happen to own Apple's tablet, there's also an iPhone version coming soon). Casey's Contraptions is in the App Store right now for US$2.99.