All of them. If you want to be decent, you should learn CLL and EG-1 at least. If you want to be really good, EG is just the beginning.
If you want to start, just learn any set. They're 6 short algs per set.
Also mods, please move this thread here:
https://www.speedsolving.com/threads/one-answer-2x2x2-question-thread.50909/