I've been through this topic before, but i'll say it again.
The BEST as far as purity and Monohydrate goes, is Creapure creatine. Prolab uses it, Universal, ON, and many other companies use their creatine.
As far as all of the others, their main thing is how they enter the body and are utilized. If you look at creatine AKG, and then pick up an exercise phys book and look at the Krebs cycle, you'll notice Alpha Keto Glutarate.
The Krebs cycle is a series of reactions which occurs in the mitochondria and results in the formation of ATP. So Creatine AKG is creatine bonded with AKG.
Right now on the market there are probably 6-7 different forms. All being something bonded with creatine for better transport.
Which one should you take? That's still in the air, and there is no solid answer. I use creatine AKG right now. I have also used Creapure before when that's all there was.