Angry Birds is a good one. There's a free version, but it has only 15 levels. Full version (99 cents) has much more levels.
Fruit Ninja is fun, too. There's also a free version for this, but there's a score limit of 125. Full version has more blades, more backrounds, no score limits, more achievements, and three more game modes.
You should also get Doodle Jump. Pretty popular and simple - tilt the iPad to jump on platforms while avoiding hazards.
These games are all 99 cents, and they all have demos (except Doodle Jump).