this site is the answer

www.wtharvey.com

I personally like using Lucas Chess for tactics training.  It has a feature for learning the tactics using the spaced repetition learning theory.  I find I retain the patterns much better using this method.  Chesstempo.com also has a spaced repetition option (it may be for gold members, which I am so I'm not 100% on whether it's available to everyone or just gold), but I like using Lucas Chess locally better.

I think it depends on what part of tactics you want to train.

For pattern recognition, I enjoy chessimo. Chess tactics server is a good alternative.

For calculation and implementation of said patterns, I use chesstempo, but chess.com is an equal alternative if you're a paying member.

More than anything, consistency is key. Just like running 5 miles one day will not reduce your weight, 5 hours of tactics training on one day will not benefit you greatly. You have to run every day to lose weight. Not 5 miles every day of course, but you get my point.