I've been in the very front two rows, the middle, and the back row, and here's my analysis:
I have to agree with what has been said before. The back seat gives you a true appreciation for the speed of the ride. You're yanked down every hill, and the first hill in the back seat is possibly one of the best experiences in the park. That being said, the back is (comparatively speaking) more 'rough' compared to the front. It's just more forceful. But I like how you get misted nicely at the end, which is refreshing, not annoyingly wet.
The second best seats are the front 4. They're really isn't a noticeable difference between the front two rows, and the views they offer are awesome, especially looking down at the midway/crypt que on the first hill. The floating airtime in the front is far from subtle (more negative G's?), but still not painfully intense. It feels like to me that your floating from the top quarter of the uphill, all the way to the bottoming out of the drop. And the front is smooth as glass. I've been other B&M's and this ride is THE SMOOTHEST ride I have ever been on. What surprised me the most is that this ride is nothing like Magnum, the only other true hypercoaster I've been on. I think Diamondback is 10x better.
Out of the middle seats I've been on, the front middle offers the most airtime and is the best option IMO.
*On a side note, I rode Diamondback on one of the first weekends and I swear they didn't use the trim break on the hill, which made that hill AMAZING but I almost blacked out on the following helix (Reminded me of Millenium Force, a ride I love, but it didn't fit the ride.) so I guess I can see why they use it.*