All of your above mentioned choices are excellent. You can safely go for the one which looks best to you. I would personally go with the Asus model as I trust its overclocking ability more compared to any other brand.
I'd also suggest you to get a CPU cooler in case you haven't thought about it already. The Corsair H80 should be awesome. Also, go for the Crucial M4 SSD. Its a tad better (refined) compared to the old C300 series. I'd recommend you to think about dropping down to the i5 2500k and think about getting the 256GB M4 model.
I recently built a new system for a cousin of mine with the Asus P8Z68-M Pro and I was very happy with how easy it went together and easy the set up was with the RAID. So I guess I would second Werner123 about the P8Z68-v Pro if you want an ATX board.
@werner - Within reason, I don't mind unnecessary price tags.
@Emperus - Thanks for the SSD advice. I kept the 2700k and went with the 256GB M4 Model. When I thought about it, I may have 'wasted' a slot and some money by only going for 128gb (of an older version too).
Please note that everything I'm talking about hasn't actually been purchased, it's currently on a wish list on Newegg.
If anyone is thinking of buying a new system, I highly recommend doing what I did. Make a wishlist about 1 month prior to purchasing. About twice a week I focus on one part and do research / ask around to find out if the part in question is the right choice.
Since I started I've made 5/6 VERY positive changes.
Okay then go for the ASUS Maximus IV Extreme-Z as asus are well known for their overclocking capabilities and stability, but you will get the same OC performance with the ASUS P8Z68-V PRO for much cheaper that's why i recommended this board, enjoy your build.