This is the app I keep coming back to and playing again and again. Its got the largest variety of really good puzzles. Its interface is great, especially if you buy the ad removal option which gives you an additional set of input options. I highly recommend it.
If you like to solve puzzles of any kind, (sudoku, minesweeper, etc.) you will love this app. Get it!
spamcow_moo about 100 Logic Games - Time Killers