dm4hire
Explorer
One true system for every game is a myth.
One true system for a player has got to be a myth too, unless you always want to do the same thing. My preferences for an SF game are different than from a fantasy game.
And if you want to build a one true game you have to do it yourself, because no one will have exactly the same preferences as you. Lots of people find games that are pretty close and tweak them, though.
As pointed out by several a "One True System" is definitely a personal goal and I think it can be achieved. BRP, Cortex, Gurps, and Savage Worlds, to name a few, definitely seem to be adaptable to different genres. The myth is definitely a "One True System" for everyone.
If you think about it the system is not the actual goal as much as the central mechanic. Each major system which serves multiple genres doesn't actually stick with a set of rules throughout all the settings as much as they share that central mechanic between them, i.e. roll d20, d100, dx+dy, and so on. Which means the real quest is finding a central mechanic with a fairly good system build around it that meets your needs.
The central mechanic is definitely the reason people cling to a specific game for so long. Start with D&D you get use to rolling d20 and the math that goes with it. It can be intimidating to learn a new dice mechanic. Throw a heavy rules system around it and you up the intimidation factor. That's probably why the OSR movement is so big right now as they tend to focus on making rules light systems. In that regard M20 is definitely climbing the heap as a universal system.