Advanced Dungeons and Dragons
It covers all the bases which I want it to and it's modularity makes it one of the easiest games to run or play as one wishes for an old school guy like me.
It can be really simple in how one runs it (just using the PHB and MM, with the combat and save tables of the DMG or OD&D), backwards compatible with OD&D if one wants to using just the MM, or the MM and PHB, or even all three, forwards compatible (2e actually had a grandfather clause) with 2e, or even BECMI or B/X if one wants to use them.
In simplicity you can focus on the base races and basic four classes to keep it simple, or expand upon that.
On the otherhand it can be extremely complex with someone trying to implement as many options in the DMG including the creation of other classes and/or the changing of existing classes and mechanics however one want to run them. In theory if running with OD&D backwards compatibility and as many of the AD&D DMG extra options, you could have almost every creature in the MM as a race option and have literally thousands of classes of your own design, all legal and ready to play for that fashion of the rules.
It can be used to run a sci-fi game, an apocalyptic future game, or even a Western.
Plus, I really like it and that's really what counts in making it the best RPG for me right now.
(PS: I reserve the right to change what I consider the best rpg at any time as this is based on my opinion at the current moment, so it may change even from moment to moment...it's not a set thing right now with this being opinion based).