This has several answers. If I'm eating at a new place or on a budget, just cheese. If a pizza place can't get a plain ol' cheese pizza right, it doesn't deserve my business, imho. haha
but once I've established that the crust, sauce, and cheese are up to snuff, I'll branch into veggies. My favorite place when I lived in Chicago was called Aroma,and I'd order half red onion & green pepper, half green olive & feta. If I'm throwing a frozen pizza in the oven, I often put those extras on top, too.
Back in the Twin Cities, I often got the greek inspired "Athena" pizza from Pizza Luce which had olives, onions, peppers, feta, and artichoke hearts.
I worked at Godfather's Pizza in high school, which resulted in a love affair with taco pizza (combining my two favorite foods???) and was stoked to find that one of the big budget chains here, Pizza School, does a "quesadilla" pizza that is quite similar, so I often order that (without mushrooms)!