The only rules set that I use with little or no alteration is Cortex. The original Cortex, not the Fate-ified Cortex Plus. The way the game is constructed, there is little that needs tweaking to work well, and the one or two issues I have with it have rarely gotten in the way of play. I did have to cobble together mass combat rules to deal with large space and land battles more quickly, but that didn’t really change the core mechanics.

I ran Marvel Heroic without any house rules, and the same with James Bond. I haven’t done any tweaking with the Ubiquity system of Hollow Earth Expedition, although i can see where the combat system needs streamlined and cleaned up badly.

I have been pretty by-th-book on Dungeons & Dragons, but that’s because I’ve got a rules lawyer in one of the players who has run it much longer than I have.