I recommend them in the following order:
ChessTempo (http://chesstempo.com/): This is definitely the top one. It has a very large database of tactics problems. Some puzzles are very difficult. Other than puzzles, it has game collections and other useful resources. A paid version is available if you are a heavy user. If you don't have much free time, this site should be suffice for all your needs.
Chessbase Tactics Trainer (http://tactics.chessbase.com/js/apps/Training/?lang=en): New. With its huge resource from Chessbase, the quality of problems is pretty good. (if you are new to chess, Chessbase is THE only software used by all professional players.)
IdeaChess (http://www.ideachess.com/): If you search in App Store for chess apps, it has a lot of them available for both iOS and Android.
Chess Tactics Server (http://chess.emrald.net/): a smaller one, only focus on tactics.
chess.com: daily tactic training, a convenient one if you use the website to play chess or watch videos.
chesskid.com: for beginners.