KO
KO is surprisingly strong considering its simplicity. However, whether it is the best system for you to learn depends on several factors. They are how much you intend to bet, what type of games you plan on playing, and how many hours you expect to play per year.
The typical scenario of limited hours, smaller stakes and mostly shoe games would point one to the KO or HI-Lo counts. However, if you are thinking about becoming a big hitter or making it your vocation rather than a recreation, you should consider a level 2 count like the Zen. And if you expect to play a lot of handheld games, then you definitely should look into multi-parameter counts like HI-OPT II or Carlson's Omega 2 count.
The gain for an ace-neutralized count can be considerable in the better games with deep penetration. A good compromise for most players is to play HI-OPT I in handheld games and HI-LO in shoe games. It is fairly easy to go back and forth between the two.
But again, if you are only playing 6 or 8 deck with unexceptional penetration or special rules, learning only the KO or HI-LO is more than adequate for most situations.