The VOIDRUNNER'S CODEX is LIVE! Explore new worlds, fight oppressive empires, fend off fearsome aliens, and wield deadly psionics with this comprehensive boxed set expansion for 5E and A5E!
I would play 2nd edition, specifically with the Player's Options books. I loved the freedom and flexibility of the customizable classes. I see a lot of that in True20, actually.
Honestly I'd play any of them, at least once just to try them, but I don't know enough about the rules differences to really base my decision on that at all. So I'm stuck with flavor, and I can't really say I care for the play style and feel of some of the older stuff I've read, so I have to go with the heyday of fluff and campaign settings...
2e: the planes had depth and the fiends had flavor.
Well, technically it's post-2E (in terms of actual publication of the book), but it's mostly a compilation of pre-2E materials. In essence, for me, it's pre-2E gaming.