Started with Hi-Lo and never switched.
The thing I like best about Hi-Lo is the incredible versatility. Wonging in, Wonging out, bet spreading, strategy changes, and shuffle tracking are all available to Hi-Lo counters with no additional steps or side counts required. There are better systems for all of the individual applications, but no one other system does a better job at all five.
The drawback is that Hi-Lo is a little more complex to learn starting off than KO or Red 7; you're most likely to make deck estimation errors which will cost you until you get that down pat.