JKD wrote:
You have to pick your application...if you want the best traditional Amiga...I say a heavily expanded A4000T.
If you want the best Amiga that was not an Amiga..Pegasos 2 G4 with MorphOS...hands down the ultimate in the evolution of the 'Amiga' platform.
Props go to AOS4 on the AmigaOne as a close second.
This post surely isn't very helpful 
I agree with all of the above except I would prefer my A4000D in a PowerTower over the A4000T from Amiga Technologies. I can't say about the Commodore A4000T as I have never seen one.
Also, I have to agree with the other poster's comments about why anyone that is not very familiar with what an Amiga is and what it can and more importantly, cannot do, would want to buy an Amiga? Other than if he wanted an original Amiga instead of running WinUAE to play games, I can't recommend to anyone to buy an Amiga anymore (and I am one of its biggest fans and collectors).
If you don't already know all about Amigas, I suggest using UAE on your current computer and reading this forum for at least a year before you go spending any money on Amiga computers (sorry AmigaKit).