While I can't say I personally like systems based around a d20, I can't have too many complaints about Advanced Dungeons and Dragons. I enjoy the complexity of the system, even, in that it prevents idiots who don't know what they're doing from just picking it up and jumping into where they don't belong.

 Add to all that, if you have a good group who knows pretty well what they're doing, it becomes a great fun to play, elaborate and expansive plots stretching far and wide, across countries and in some cases planes or even other worlds.

 Truly a great fun.