Basically this is exactly what it says on the tin. Of course, you also have to include One Piece characters into this, as well. Anyways, let me get started.
Basically, I'll list down a few crossover fights between X character from One Piece and X character from another anime, and give your opinion on who would take the upper hand in the fight. You give your opinion on those. Then you can write down your own crossover fights in the comments section (not all of the opponents have to be from anime or manga).
So, who do you think would win in these fights?:
Crocodile vs. Gaara (Naruto)
Rob Lucci (all forms) vs. Quilge Opie (Bleach)
Post-timeskip Luffy vs. Ulquiorra (Bleach)
Luffy vs. Natsu (Fairy Tail)
Caesar vs. Stein (Soul Eater - both mad scientists, so why not?)
Okay, so I'd like to see your opinion on who'd win all those fights. Don't be afraid to make some of your own. Cheers.