Well if we go by the one we used the most it's probably "4d6 and drop the lowest" for rolling up D&D characters going back to sometime in the 80's.
Other than that we don't usually need a ton of house rules, at least not a codified set of them. Lord knows Pathfinder has enough rules already. Savage Worlds works pretty well as-written. M&M does too.
I've played around with some tweaks for ICONS here. It's the kind of game that inspires some mechanical exploration.
All of my rules-related posts are here so I'll just let that stand as the rest of my entry for this one.