I support Luffy vs. Usopp. Because fights don't have to be about big techniques or strong opponents. Every punch and dial they were hit by must've hurt so much more just because it was from their best friend, not to mention how much it must've hurt inside to punch your friend in the face with all you've got. There was never such mental torment and emotion behind a fight again (okay, maybe Luffy vs. Lucci, but in a much different way) and thats, in my opinion what makes this fight great. It wasn't filled with tension because you were unsure who was gonna win; you knew from the start that no one would, really. It's the pure emotion of friends being ripped apart against their will that makes the fight intense all the way through. Watching this in the anime the first time, I won't lie, I had tears in my eyes.
Not saying that it wasn't at all a good fight in terms of fighting either, because Usopp really got to show off his "Usopp Tactics" (my nomination for favorite non-DF ability) against someone he knew how to fight. But where that style of fighting is usually used as comedy in the series, that was all turned around in this fight. Every lie, every ketchup stain and egg to the face which would normally be funny was just seen as tragic attempts to best his captain who he knew was much stronger than him - but surprisingly, it worked quite well. Not to the point where you were ever unsure that he'd be beaten by Luffy, though. But again, in a fight like this, no one wins.
It's been my favorite fight since I first saw it with eyes filled with tears.