In the day of Open System RPGs, This answer for me is the CYPHER System. Followed by the close second of FATE Accelerated & FATE.
On a personal level, I love the way that the CYPHER System plays both plays and runs. I find that the system loans itself very intuitively to mimic many other RPG settings with fall less bulk. That is an accomplishment that leaves a GM wanting to find more settings to sick the Cypher ruleset with.
FATE & FATE Accelerated fall in second for me on this because of the amount of front end work that goes into preparation for a game of FATE. Both Cypher and FATE have some front end work but FATE requires more so it is edged out just slightly in my opinion.
Runkle Plays Games 