Hmm. This is actually a really tough question for me, because I'm not sure I have one. I haven't had one particular one-on-one fight that I look forward to with great delight in any of these movies. I guess I'll go with Sam vs. Shelob because it's such a heroic moment, and Sam really comes into his own there.
Also, I'm a big fan of killing spiders, or even just wounding and driving them away, so on a personal level, I'm happy Sam dispatches Shelob :-)