Well, I don't have a lot- my best vader is TSC Vader after all- but I'd say the best-looking and the one I've heard the most good about is the Evolutions Vader. He looks darn good, and big- appropriately big. The sculpting is great and you can pose him however you'd like.
VOTC Vader looks like a good enough figure, but the sculpting seems wrong in a number of ways. Most glaring is his slightly mis-sized/mis-shaped helmet- not sure exactly what's wrong with it, but a comparison shot between him and the 500th/TSC and Evolutions Vaders shows what I mean.
If he's never going to get moved much though... have you considered a 500th or TSC Vader? The TSC ones seem to be all over the place, and the sculptwork is all very good, and the cape's flow can be controlled if you know how to handle it and discreetly try to tuck it under the arms and such, unlike a certain ROTS Vader
and his "I'm Batman!" cape (and pose).
If you'll forgive me liking again to Rebelscum's photo archive (look at it this way: you're stealing their bandwidth!), I think this comparison shot
shows your probable best choices.
JD doesn't happen to have a comparison shot of these four Vaders up, but they do have some rather nice archives of each for your further comparison: VOTC Vader
- supposedly the "ultimate" Vader, but, angle cuts and such... and a little wacky sculpting...2006 TSC Hoth Vader
- rerelease of the 500th Vader and soon to be rerereleased in the Tatooine Wave, probably cheapest and easiest to find (check your local Wal Mart and Target
- Arguably the Ultimate Vader, though he has right-hand grip issues according to some and lacks VOTC Vader's ball-jointed head and belt saber hilt; he makes up for it with lots of ball-joints elsewhere, great sculpting, and cool soft-goods
500th Vader doesn't seem to be in the JD archives; basically he's TSC Vader with a different paintjob on the chest/shoulder armor, a second removeable right hand that resembles the left, and a meditation chamber which can remove/replace his helmet via lever.
Hope that helps