onions but that's just because onions are a disgusting abomination that deserve death
I haven't seen many weird pizza combinations. I love pineapple on pizza but haven't had it in a long time. If I had to choose, I'd say seafood. Even though I love it by itself, the flavors would likely seep into the slice (like they do with onions, blegh) and seafood bread sounds gross.
now ima go hide before the onion lovers come get me