So for example, in Mario, Bowser always has a lot more abilities and powers then Mario, he can breathe fire, he's much bigger, he can throw axes etc. so that wouldn't count, but Mario Vs. a Koopa Kid would be the type of fight I'm talking about. Nothing in which the Boss is too big, powerful etc. Also I'm not including multiplayer, since most multiplayer games can be considered duels, and traditional fighting games are also out since fighting games are essentially duels.
Metal Gear Solid Series
The Metal Gear Solid series has some amazing duels. I've only played 1 & 2 unfortunately, but in those two alone there were tons of one on one's. Psycho Mantis, Fortune, Vamp, Revolver Ocelot etc.
Solid Snake and Raiden fight all kinds of villians having weird powers with as much variance as the X-Men. The Duels in the Metal Gear Solid Series are top notch, and worth a look.
Raiden VS Vamp (Metal Gear Solid 2)
Solid Snake VS Screaming Mantis (Metal Gear Solid 4)
Mad world is possibly my favorite game on the Wii. In Madworld, the protagonist (if you can call him that), Jack, is basically on a TV show where the objective of the "game" is to be the number one tough guy in Vatican City. You rise through the ranks, picking off one contestant after another, until you've killed everyone in your way on your quest to #1.
Jack VS Jude the Dude (Madworld)
Madworld is full of duels, every boss is a one on one with some sort of unique power or ability. Jack's ability? He has a chainsaw strapped to his arm, and he's a complete bada**. This is a game where I played the bosses over and over again, because amazing duels is what this game is all about.
Warning: This is the final boss of Madworld, if you intend on playing it, don't watch it, if you don't care, prepare for one of the coolest fights in gaming.
Final Fantasy: Dissidia
Yes, I know it's a fighting game, but I did say "traditional" fighting game, and Dissidia is far from that. Final Fantasy Dissidia is just pure gaming awesomeness. The duels are so engaging and fun that it gives you that "high and mighty" feeling when you win a hard battle. All of Final Fantasy's main protagonists and antagonists go head to head with all their powers to see what rules all; the light or the darkness.
With hundreds of equitable item, weapons, and clothing, and a huge synthesis system, the customization is endless when your creating the perfect fighter. With the fast paced incredible soundtrack, and speed of the battle, Dissidia will engage you in many many ways.
I personally recorded these myself, since Dissidia makes it pretty easy to get replay videos off your PSP and onto your computer, if you've never seen any footage of Dissidia, now is the time to do it.
Warrior of Light VS Garland
The Warrior of Light Vs. Garland, both from the first Final Fantasy. The interesting thing about this fight is that I actually won with zero HP, Garland hit me right as I was doing my finisher and I somehow came out ahead.
Cloud VS Ultimecia
Cloud, from Final Fantasy VII, VS Ultimecia, from Final Fantasy VIII. Ultimecia had me on the ropes the whole time, but in the end, I pulled ahead and beat her.
Onion Knight VS Sephiroth
Onion Knight, from Final Fantasy III, VS Sephiroth, from Final Fantasy VII. Onion Knight is actually my favorite character, he has a ton of magical abilities, plus physical attacks, and he has 2 special moves, one as the "Ninja" class and the other as a Red Mage, which are both demonstrated in this video.
Bomberman 64/ The Second Attack
Yes, Bomberman 64 again, I know I write about it a lot, but its just that good of a game. Bomberman 64/ The Second Attack isn't like any other Bomberman game out there. These two amazing platformers have an awesome story, great gameplay, and of course, adrenaline filled duels.
Bomberman VS Regulus (Bomberman 64)
Regulus' dash attack is a one hit KO
Bomberman VS Altair (Bomberman 64)
"You wanna fight me?"
Bomberman VS Zohina (Bomberman 64: The Second Attack)
Warning: Spoiler @ 3:00 in this video, if you've never played "The Second Attack" but you have played Bomberman 64, this will spoil something about the plot of the sequel.
If you're a gamer you could have seen this coming from miles away. Of course, no game has greater duels than the Megaman series. The robot masters basically have every power under the sun (especially Solarman) and there are sure plenty of them. With well over 100 robot masters, Megaman is the perfect example of how a proper duel should be (but only if you use the Megabuster, of course).
Megaman 2 Boss Run
Megaman 9 Boss Run
So, any games I'm missing? I haven't played No More Hero's yet or I'm sure that would have made the list. Anyone have any games with awesome duels? Let me know in the comments.