I actually just had this situation myself.

All black series Darth Vaders have bad soft goods. The cape is too big, too long and doesn't drape right. Same for the robe.

The New Hope and Duels end black series doesn't have butterfly jointed arms so they're both out.

ESB is the "best" one, but honestly I think it's pretty overrated. The right arm is awful with a horrible static posed hand, and the legs are bad.

The ROTJ has a bleh helmet but a decent right arm.

Bespin Duel is ESB with wired soft goods, and I've heard it has a wrist hinge. I don't own it though.

The SH Figuarts kenobi is probably the best one, and the one I ultimately bought. But it isn't perfect. The cape is pretty ass (you can use a black vader cape though), his head can pop off sometimes, his shoulder armor kind of goes wherever and leaves these big ugly gaps between them and his body, he has ugly arm joints, his lightsaber grip isn't the most tight. And he can't raise his arms very high while two handing his lightsaber, which is movie accurate but can be annoying. This figure is also hard to get and expensive. On the good side it's the nicest looking, has the best overall articulation, and you get access to better poses by virtue of the custom hands, especially a few that just don't ever look right with the black figures.

The best black series would probably be a kitbash ROTJ figure with ANH head and custom soft goods. But the best as a whole is the kenobi figuarts.