Which Star Trek series, was the most diverse, the most "Flexible" from it's original design, I'm going to have to say Voyager.. After all we met all those Alien Races in the Delta Quadrant..

I'd have to disagree and say Deep Space Nine. Although you'd think the premise of a space station doesn't lend itself to flexibility, we've seen DS9 (the series) go from station-based stories, to political intrigue on Earth, a Cardassian resistance movement on Cardassia, attempted genocide by rogue elements of the Federation, and so on and so forth. Deep Space Nine was nothing if not dynamic! Anyone else have thoughts?

I agree DS9, it's the one that ran the gambat and covered the most ground with what was possible.

I don't see what meeting a good number of alien races has to do with diversity or flexibility. If anything, Voyager is possibly the MOST limited sereis, with Enterprise just behind, because of the lack of a Starfleet infrastructure in the area of space where it travelled, the inability to feature friends or loved ones from back home without flashbacks or contriviances, the difficulty in doing ongoing continuity when the ship is supposedly passing through regions of space where it will not return, etc.

DS9, having the option of station-set shows or off-station shows that could be done by using runabouts, the Defiant, and/or the wormhole, DID have the greater ability to run the gamut of stories, as Brian said.

With Voyager, we had "Aliens of the week," including "Hard-headed aliens of the week." With DS9, we had cultures of the week... or month or season.

No contest: TOS!
No lengthy arcs, and hardly any recurring characters. Just a series of short stories connected only by the cast and the overarching theme.

(Unless, of course, "flexible" means "not bound by canon." That would be VOY.)