For me, the greatest rule of any game has to be in Stargate: SG-1. It has a simple, yet amazingly deep character system despite being your basic D20 system. Your first choice is what species do you want to play, human, Asgard, Jaffa, or Tok'ra. The Asgard, Jaffa, and Tok'ra each have four options already, but it's the humans that have the most variety.
First you choose if you want to play a US military member, a civilian scientest, an NID infiltrator, or a Russian Stargate member. If you choose to play in the military, then you get to choose which branch (Army, Navy, Air Force, or Marines) and each branch gives plusses and minuses to your stats. THEN each branch has four options (enlisted, officer, special forces, or technical specialist), each of which gives special skills or feats. THEN you choose your classes, of which there are six basic classes and six prestiege classes.
Add to that literally hundreds of feats and background options, and you get a game that is amazingly flexable in its character creation despite only having twelve total classes and six ability scores. I've never come across another game that's nearly as flexible.