BRP

The system I keep returning to is Basic Role Playing, Chaosium's house system. This is the core game system behind a variety of games produced by Chaosium over the years, including:

• RuneQuest
• Stormbringer
• Call of Cthulhu
• Worlds of Wonder
• Ringworld
• ElfQuest
• SuperWorld
• Hawkmoon
• Nephilim
• Elric!

Chaosium's King Arthur Pendragon is considered by many to be closely related to BRP.

Many of these games are no longer in print, though some are available for .PDF download, and most show up from time to time for sale or auction on the web (such as on eBay).