The problem with starting a discussion with "best" is that it depends heavily on what criteria you use, and there is a lot of subjectivity around.

But you can take the components and see what fits. All have a meaning, a best scenario to use them. And if well the line dividing the devices for some kind of uses could be blurry (i.e. a tablet with bluetooth keyboard vs notebook/netbook, i'd bet netvertibles could simplify things there) you have cases where is clear the superiority of one solution or another.

I would say that the 2 extremes of the scale can't be avoided. The mobile phone/computer as in the N900 or future devices with that kind of functionality in around that size is a must for a big while. And the desktop computer, as something with separate displays, input devices and central cpu where you can have flexibility to adapt and upgrade, and is meant to be used in a very specific and comfortable spot for a full experience, is a must too. And the middle ground (i.e. at least a netvertible) should be there too.

In theory most of the roles could be done with an N900 or smaller sized device with the right accessories, but not sure if there is some kind of wireless/secure communication technology that could communicate fast between the cpu and input/output device like screen or input cameras, so you need at least cables if not being directly integrated with the cpu, and there you give up mobility or have different devices for different scenarios.