Plain, Because the Set doesn't have anything covering the Technic on that tail, the Pizza wouldn't have any Toppings.

I can't think of an exact combination off the top of my head. I would think it'd be some experimental pizza using an odd combination of ingredients which, on their own, are pretty good but when they come together they're kinda bleh.

As for the others:

Skull Warrior is peperoni, a little borning but a classic.

Skull Basher is Hawaiian, a bit different but still an acceptable favorite.

Skull Slicer is like a bit of everything, which sounds good but then it turns out to be pretty messy when you get the end product.

Skull Grinder is cheesy bread, you're only ordering him because he comes with the Mask of Creation.

None pizza, left beef.

If Skull Scorpio were a pizza, he'd be a pizza with grammar lessons on top to teach you how to properly use the subjunctive mood.

Scalawag.