As I understand it, this is a tournament to determine who has the best belly, not who shows it off the most. Victoria's belly is lovely, but Kaley possesses the type of belly that other women would die for - not only is it tight and toned, it's long, and thus is better suited for low-rise pants (like what she wore in her 8 Simple Rules days). Even when her bellybutton isn't completely visible, her lower belly still goes on for several miles, long enough to get any belly enthusiast hooked. And if we're going to talk about just the bellybutton, Victoria's is nice, but it doesn't command my attention the same way Kaley's does. Kaley's innie is more detailed and distinct, which makes it easier to identify and remember, and it has evolved just enough over time so that it's always interesting to look at. I grant Victoria credit for her prolific belly-baring career, but when you look at her belly next to Kaley's, it should be clear which one is more impressive.
As if you hadn't already guessed, Kaley Cuoco gets my final vote.