I like Luke/Vader 2 in ROTJ. When Luke gets all pissed, and the choir kicks in, that gives me chills.
I'll say I think the Obi/Vader 1 in ROTS is great... up until they start swining on vines over lava and things just get ridiculous. I think the moves, and the speed, and the saber and force attacks up until then are awesome. All that CGI near the end felt like gilding a lilly.