After hearing Pumpkinqueens answer to the crossover question, definitely FMA!

and how

Ninjas and Pokemon *nods head* I am sure a lot more nerds would watch pokemon if there were more ninjas (besides Koga, Janine, and Kya). Nerds tend to like ninjas :D

closer in common then the other series IMO

Probably Bakugan

Digimon digital monsters digimon are the champions ^_^ I wanna be the very best like no one ever was chang into digital champions to change the digital world! To catch themn is my real test to train them is my cause DIGIMON POKEMON (simotaniously digimon and pokemon at the end)

Probably Digimon, digital monsters..and Pokemon Pocket Monsters,I would think it would be an interesting crossover...but I'm pretty sure the Digimon talk..if I'm remembering correctly anyway...while Rocket Gang's Nyarth is the only (main) Pokemon that talks..but he says "Nyar" in the beginning of each sentence he says so it might be hard but still I think Digimon would be the best crossover with Pokemon.

I think it would be horrible. They are rivals with each other

LOL FMA and Pokemon would be awesome. XD

Oh and this is what animegrl52p meant by my answer, I said:

Edward: *points at Ash* Look, I'm not the shortest person anymore!!! :D
Ash: Hey!!
Brock: "looks at Envy" HOLY MILTANK!!!! Now, that's a a woman!!!
Envy: I'M A BOY YOU IDIOT!!!!
Jessie: Then why are you wearing a skirt?
Envy: Because it makes me look hot.
James: I say it does!....Can I borrow it?
Envy: No you can't, are you all crazy here or something!!
Pikachu: Pikachu!
Envy: *eye twiches*

that would be the dumbest thing that pokemon could ever do

^ continue is the dumbest thing pokemon can ever do.