Curious if anyone knows which smart cover is the most popular?

You'd think it would be the smartest one, but experience has show that people prefer the one's that are only slightly smarter than average. Less intimidating.

Only the Apple branded smart covers are really Smart Covers, and there you've only got a choice of poly, leather, and some colors. I'm guessing you're question is broader and you want opinions on covers that have the same magnetic personality; able to lock/unlock the screen.

Unfortunately all I have to offer is this wise-cracking post, and my one, unimaginative, choice. Hopefully some members with better rounded experience will come along and give you actual advice.

I've been using the basic Apple Smart cover, poly, gray. Along with a matching back cover by Hypershield I find it adequate for toting around the house and in the car (usually under a newspaper or blanket to discourage theft and/or sun damage). If I were more mobile with it I'd probably want something tougher. If I was more business I'd want something a bit classier.

I am using the basic apple 'red' and "orange' smart cover. And it looks great with my white iPad.

I have a white ipad and I use the Polyurethane black smart cover. It's very nice.