All categories have the perks/drawbacks. It's difficult to pick the single best.
Vintage- Everyone who had these as a kid loves em. They were the coolest figures, before they all had names, before they had histories, they were just TOYS
POTF2- Gets a bad rap because it has some of the worst sculpted figures made to date. Yet, it also has some really good and innovative ones (Slave Leia, Helmetless Vader), and blazed the way for all the later lines.
E1- Well, not much to say about it, but it gave us all of our prequel figures at the time, and had it good, bad, and ugly. On the up side, it gave us Horox Ryyder and the awesome Maul from the 3-pack.
POTJ- Great series. Gave us some of the fan favorites. Leftovers from episode one that might otherwise have been scrapped, and the first of the real New sculpts of the POTF2 figures.
Saga- In my opinion, a great series. I like the poses, and some of the action features. I'm jaded because I feel the best way to display new figures is in dioramas, and these poses/features lead to better dioramas. However, this line also has some of the most detailed sulpts, and best of all, most figures come with tons of accessories. You can't say the Exploding Radar Dish, a free Tri-Pod cannon, storage crates, or even the upcoming bases aren't pretty nice.
I voted for Saga, since I find it to be a lot of fun. However, it builds on the previous sets, and if I could only keep one line, I'd be hard pressed to pick a definitive series. (Vintage has the advantage of nostalgia)