I would definitely go with Zio and Lashiec. Both can take and dish out a beating and "Laughter" is one of the best boss songs in the game. Not to mention that if you played PS1 first, both battles are wonderful tributes. (I literally jumped the first time Zio cast Nightmare)
Dark Force 3 is kinda neat too. There's a coolness factor associated with having a party member turn on you and use the skills that used to be at your disposal against you. I only wish DF3 would have had the same open mouth with sharp teeth that the other incarnations of Dark Force (bar the PS3 version) had.