D&D 3.5 and 3.0 here, with both core, published and custom campeign settings, using modified rules both approved (Unearthed Arcana) and un-aproved. I will not touch 4.0 with a 10 foot pole reguardless of whether the talented and hilarious sell-outs from Penny Arcade are paid to like it or not.
Star Wars d20, D20 Modern/Future/Past/Apocolypse are also good.
Me and my friends have also worked for a number of years on turning the brilliant yet half finished "Inquisitor" game by GW into a roleplaying game fit for kings, which so far trumps the new "offical" rp, Dark Heresy, although it is rather good.
Edit: Warhamemr Fantasy RP 1st and Second ed are also favorites of mine-How could I forget?