That's a really interesting topic. Guess I'll spent some time here. If price is not a concern for you, well, check out the following list which I think is great and useful. For me, that is.
# Puzzle- Labyrinth [$6.99]
-> Great to experience the power of accelerometer and the stability of your hand. I clear 1 level per day. With the option to download other user custom level pack, it will keep you entertained for loooong.
- Smart Sokoban [$0.99]
-> By far the simplest and best sokoban game.
- Know How [$0.99]
-> Yet another puzzle game which require you to push the treasure box to a point. Nice 3D graphic and smooth game play.
- Loops of Zen [$1.99]
-> A very interesting puzzle game where you complete each level by closing the loops. Japanese are really good at developing such game.
- Frenzie [$1.99]
- Mines in Space [$1.99]
-> Simply the best minesweeper game you can find! You can play it for years and yet fully unlock the achievements.
- Hydrotilt [$2.99]
- TikiTowers [$0.99]
-> Build your bridges so that the monkeys can safety reach the finish point.
# Racing- Crash Bandicoot Nitro Kart 3D [$5.99]
-> There's been issue about it lagging in some scene, but it's still playable
- Fastlane Street Racing [$3.99]
-> A nice racing game where you can actually feel the speed sensation. It's pretty hard at first play. No radar that shows the layout of the track really make it a realistic racing game.
- Snail Mail [$1.99]
-> Smooth game play and nice graphic. You can't miss this one out.
# Board Games- Xiangqi [$0.99]
-> Features a beautiful water art. It's a chinese chess game. For me it's just perfect.
- Monopoly: Here and Now World Edition [$7.99]
-> Buy this if you love to play Monopoly but don't have anyone to play with.
- Scrabble [$4.99]
-> The name says it all.
- UNO [$5.99]
# Brain Training- Brain Toot [$0.99]
-> Simple and addictive brain training game
# Leisure/Relaxing- Scoops [$1.99]
-> A pretty cute game.
- Sneezies [$0.99]
-> If you like cute stuff you definitely need to get this. The sneezing sound is what appealing to me.
- Buggled [$0.99]
- Giant Jigsaw [$0.99]
-> A 3D featured jigsaw game. Truly one of a kind. Be aware of your battery bar. It drains out pretty fast with this game. It should be understandable considering the 3D engine it uses.
- SPiN [$4.99]
-> Silhouette matching game. Think you can think in multiple angle?
# Tower Defense- FieldRunners [$4.99]
-> I won't say much about this. My 2nd TD game.
- 7 Cities TD [$4.99]
-> Full featured TD game.
- The Creeps [$1.99]
-> With its fast animation and nice looking sprite, this game is a steal at $1.99.
# Sports- Midnight Pool [$4.99]
-> The aiming feature is great for beginners. You can also unlock more characters. Vivid and colorful environment.
# Memory- Matches Plus [$1.99]
-> Simple and elegant matches game.
# Action/Adventure- Chopper [$0.99]
- Amateur Surgeon [$4.99]
-> Don't expect nudity here.

- Bike or Die 2 [$6.99]
- Dr. Awesome [$0.99]
-> It uses your contact as your in-game patient. Really thoughtful.
- Space Ninja [$3.99]
-> A nicely twisted game whereby you dodge enemies bullets instead of shooting them.
# Vision- Mad Shells [$1.99]
-> Think you have a fast eye? They even have online leaderboard where you can compete your score with anyone. So far I can only reached LV29.
# Role-Playing Game- Arvale: Journey of Illusion [$2.99]
# Platform Games- KamiCrazy [$1.99]
-> Guide your troops from point A to point B.
Whew. Finish at last.