As hilarious as Water-Rotom looks, and as adorable as Flying-Rotom looks, I still don't really like any of them haha. They are just a bit too cartoony for me, even though it got better when I realized the canon explanation for their forms. At first I thought they were actually pokémon who were born to look like machines :p
But the normal Rotom is my favorite I guess. I like pokémon who look like that, for some reason, such as Castform and Masquerain too. Rotom has the same kind of tiny-ball-with-wings-thingy going on. All this is purely design-wise though. I haven't played with Rotom in a game or competitively.