Agree with
@awiredprairie --- what are you defining as "flexible"?
Are you wanting something that can handle lots of varying power levels (zero to demi-god)?
Are you wanting something with fast, easy prep so you can focus on story?
Do you want something that lets you move between settings / genres easily?
Do you want something that is more "flexible" in providing player narrative control instead of simple binary pass/fail action resolution?
My default go-to system for everything is Savage Worlds, and in my opinion it's much more flexible and easier to learn than D&D.
The first question I ask myself when considering what system to use for a particular campaign is, "Is there anything that will work better for this setting than Savage Worlds?" It does so many things well, with an elegant, intuitive system.
Edit: Forgot to mention that Savage Worlds has incredibly easy prep. You can easily prep sessions in 15-30 minutes, and if push comes to shove and you need to totally wing it, it'll generally hold together just fine. You may not get the epitome of roleplaying excellence with zero prep, but the game won't come to a screeching halt.
But ---- it doesn't support as wide a range of power levels; its default mode is functionally equivalent to D&D levels 2-9.
Also, Savage Worlds produces a very specific feel in play, even when moving between genres. If you don't like the way it feels in one setting, switching to another setting isn't going to change the basic feel of gameplay. I think most systems are like this, so it's not necessarily a black mark, but something to be aware of.
