Sielle: I laugh at anyone who thinks any edition of D&D is flexible! <Looks over at his collection of GURPS books while working on a PhD in mathematics so he can make a vehicle in said system>
KoolZoid: Well, it's flexible as far as editions of D&D goes, and that's what the OP wanted :)
I've nothing agains GURPS myself, although I favour the HERO system for letting you do what you like (plus I love my d6s) :P
Yea, I was just being snarky. I see all versions of D&D as combat simulators with non-combat role playing having to take place outside of the framework of the rules. Can still be fun as long as your DM/GM is good.
Can't say I've actually tried the Hero system, but if it's massive D6's you're looking for, might I introduce you to Shadowrun. ;)
roninnogitsune: Avoid 2nd edition AD&D, it's filled with bad complexity that can bod down the game, non spell casters have very little depth in their choices and there's stupid limitations to Races and Classes that are only there to enforce cliches. Oh! and the different experience tables make it a chore to balance as a GM. It's an overrated mess.
While I agree with avoiding it as far as a rule system goes, those AD&D 2nd edition handbooks (the brown faux leather covers) can't be beat for background and setting information. All the flavor text they put into those make them worth owning on their own.