The final battle at the end of
Seven Samurai is, for my money, the best fight sequence in cinema history. Just for the complexity of it, and the innovative use of multiple cameras to film it; it redefined the way action scenes were filmed. Plus, the conditions for filming were astonishingly bad (they had to break through a layer of ice to get to the mud underneath), and the rain is torrential throughout, making for one hell of a visually impressive showdown. Add to the fact that it is astonishingly moving, the ending in particular, and you have one of the best, and certainly most influential fight scenes ever: any time you see a film where the final battle takes place during a downpour, that's
Seven Samurai's inspiration right there.
Other great ones include the oft-mentioned
Oldboy, and another personal favourite of mine, the fight between Nameless and Broken Sword in
Hero. It's Jet Li and Tony Leung, assisted by wires, fighting on the surface of a lake. It's unbelievably gorgeous to look at.
Also, the duel between the Man in Black and Inigo Montoya in
The Princess Bride. A lot of what makes this one so enjoyable is the fact that the combatants have nothing against each other: they're both fencing masters, and simply want to enjoy the opportunity to fight someone as good as they are. It's not a fight driven by anger, but by a desire to celebrate their ability, which makes for a fight scene that's much more funny than most, and very entertaining.