Angry Birds = over 165 levels, bonus levels, high replay value to master each level.. still constantly being updated to add even more levels.. challenging and very well polished.. Fruit Ninja = simple fun of slashing fruits, skill required, highscore openfeint, achievements, also very well polished
I've actually been debating on Angry birds. Should I get it even if I already have Crush the castle and Castle Smasher?
Besides Angry Birds and Doodle Jump.. I'd add Run Run Sheep.. a pretty cool puzzle game. very similar to Chu Chu Rocket. It haven't been updated for ages but with its 100 Levels, it will take you some time to make it through!
For iPad - I would throw Air Assault and Conquist into the mix. I've gotten a ton of use out of these aps. I'm espically impressed how complete AA is for a dollar. It's better than a lot of shmups i've played on XBLA lately...and for a fraction of the price.
I'm new to the whole world of apps, but so far my vote would go to Angry Birds. I can't remember ever getting so much fun out of a measly $.99 purchase. Ravensword was a great one too.
A very broad question. Are you referring to only games that are permanently $0.99? Or do you include games that are currently, or have been, on sale for $0.99?