I've always thought Godfathers was the best, mainly because no refried beans on it. They have an actual taco sauce instead of normal pizza sauce, then beef. The cheese is a mix of cheddar and mozza...then it goes in the oven.
Basically, a beef pizza with different sauce up until you put the taco toppings on it once pulled from the oven.
Pull it, then lettuce/tomatoes/onions, more cheddar cheese - then the taco chips crushed up (me personally, I detest taco ships on them). Served with a packet of taco sauce Taco Bell style for each piece.
I remember in the early 80's, it was like a gift from pizza heaven when eaten right out of the oven. It really hasn't changed since when it was introduced in the late 70's (when I worked there as a 17 year old).
Used to get them quite often, but nowadays finding a Godfather's you can actually sit in and eat is becoming hard to find - and I've always thought taco pizza doesn't deliver all that well. Like someone said...taco pizza once cold, you can't re-heat it really.