Round 1: Angol Moa vs. Excel
This is no contest... Moa would just cause Armageddon and destroy the world. Granted, the Great Will of the Macrocosm would probably restore everything later, but the point will have been made. Excel is the type of person who would embrace her fated destruction rather than try to stop it. Well, accept it if not embrace it.
Round 2: Haruhi Suzumiya vs. Himeko Katagiri
Wow. Talk about a clash of titans. On one hand, you have the ultimate power over the fate of the universe and limitless possibilities. On the other hand, you have... well, pretty much the same thing, but for comic effect. It's the power of "only when it's funny" versus the power of "it's funny, but we take it seriously". Still, Himeko's technically an ordinary girl who's just weird and has tons of energy, and who hangs around with exactly the sort of people Haruhi wants to meet. Particularly aliens. I think they'd probably just go about their own business and ignore each other completely.
Round 3: Hinaichigo vs. Keroberos
It's tough to call this one. Technically speaking, Hinaichigo is a doll with the power to control other toys, and Keroberos is the Beast of the Sun, with the power to control earth and fire. On the other hand, it would be doing Hinaichigo an injustice to count her out of it just because she's little. Still, out of all the Rozen Maidens, I think she's the least likely to beat pretty much anyone in a fight, especially a flying, fire-breathing, legendary beast. Barring some fluke of narrative skill, I think Kero wins this round. (Maybe something like she tricks him with her cuteness. That could certainly work.)
Round 4: Lola vs. Mackenzie
"Hey, are you trying to hit me?"
(Why does she keep rocking back and forth? Is that supposed to psyche me up for golf?)
I can't picture either of these two actually fighting. More likely, Lola will hire Mackenzie to figure out where a missing ball went, and it'll turn out that a talking polar bear stole it.
Round 5: Meilin Li vs. Mesousa
(Meilin's the one crying into Tomoyo's lap.)
The question isn't who will win. The question is how long the beating will continue before Meilin leaves Mesousa's bloody corpse to rot and he has to be replaced with a new Mesousa.
Round 6: Miharu Sena Kanaka vs. Millefeuille
(Millefeuille's on the left side of the picture.)
Milfie is incredibly lucky. That alone pretty much assures her an accidental victory in any sort of contest between these two... unless it's a contest of sex appeal, but I don't think we're going to go there.
Round 7: Mikuru Asahina vs. Mint Blancmanche
(Mikuru's trying unsuccessfully to hide behind Haruhi.)
Mint likes to dress up. A lot. Mikuru hates it, but goes along with it because it's what she has to do. (It's complicated.) I imagine that any fight between the two would require them to be in costume, and Mint takes the victory because she's more comfortable with the situation. Also, Mint can be a callous little bitch when her back's to the wall, while Mikuru tends to be clumsy. The math does itself.
Round 8: Potamos vs. Sasshi Imamiya
At last, a tough one. The power of water against the power of whatever dimension the fight is taking place in. I suppose that, since Sasshi doesn't really have a home-field advantage in "his" dimension, we'll have to do this in the Wedding Peach universe, and that's a magical girl series. Sasshi didn't take part in the magical girl episode of Abenobashi, and is unlikely to have the sorts of powers that the Love Angels have. Then the question is whether Potamos might be attracted to him enough to refuse to fight. Judging by the fact that he's Sasshi, I'm going to call that a no. Potamos wipes the floor with him... pretty literally, I'd imagine.
Round 9: Shinku vs. Sugar
Sugar's a lover, not a fighter. Also, her power over snow doesn't work very well. Shinku, on the other hand, isn't fooled by cuteness and will do whatever it takes to win. There's a very real possibility that Shinku wouldn't even be able to see Sugar, in which case I suppose it's a draw until Sugar learns to make snow and can trap Shinku in a blizzard or something, but given that Shinku is herself a supernatural entity, I figure she can see and destroy Sugar with no problem.
Round 10: Sylar vs. Tomoyo Daidouji
WHYYYYYYYYYYYYYYYYYYYYYYY? Why have you implanted in my head the image of Sylar eating Tomoyo's brain? Why would you do that? It's not right! It's not right! Someone eat MY brain and never let me think about this again!
I'm thoroughly disturbed.