If all iMacs had the same CPU, then I'd say the combo drive iMac would be the best seller. But with the current lineup, I'd say the superdrive model is probably going to sell the most. It's difficult to call, because the price is a bit high for the superdrive model, but I think people are spec whores as was already mentioned.
The big unknown factor is not demand, but supply. If Apple doesn't get their sh!t together soon, then their quarter profits are going to be bad, very bad. People have a tendency to cancel orders when they take too long.
I know of at least one person who bought a Wintel because they didn't want to wait on the iMac. Apple really fu[ked up by announcing the iMacs before they were ready to meet demand. I think Jobs just wanted something to talk about at MWSF and w/o the iMac the show would have been a failure.