Melee and Wizard.... 'nough said.
+1, QFT!
Having said that, it could be argued that Melee and Wizard are drop-in combat systems for other RPGs, something Steve Jackson himself says he intended with them. Death Test and Death Test II do a pretty good job of providing RPG content for the system each in less than 32 pages though, and soloable to boot! 15-16 year old me would certainly have argued with you if you claimed that they weren't RPGs.
Even in the recently re-issued incarnation, with the full
In the Labyrinth book and the published adventures, the system is considerably lighter (and MUCH less expensive) than other systems.