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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.