Honestly said, it's wrong to ask "Which the highest limit you've been playing?", because it's more rational to ask "Which the highest limit you are crushing?" I mean, pretty much most of us can try ourselves at higher stakes to find out themselves as a bait for the sharks. I mean, i don't want to tell, that someone is a fish here, but obviously the fact is with every each limit you go up, the more regular/profi and less recreational players are there. So if you feel yourself as a master of poker sitting in fishtank on micros, on lows and mids it's more like a fight for a couples of fish among regs. At some limits and times, if you have unluck you can play only with the regs, which i recommend to nobody. As at the end of the day, we are grinding for money and not for showing off, that we are the best reg at the table of regs... The conclusion of all said above, please make really sure you are and your
bankroll ready for that level up and you are adequately reflective beating that limit.
p.s. I would rather play more tables i'm crushing at lower limits, than playing fewer tables on higher and loosing.