For a Mac it's easy: 17" PowerBook 1.25GHz w/1GB RAM etc... (the 15" is OK if you are going to use it mostly connected to an external display).
For a PC: I haven't been in the market for one lately (all Mac clients these days) so I don't know the current models, but I have always found the IBM ThinkPads (professional models not the CompUSA versions) to be MUCH better built than any of the others I have had experience with (Dell, Sony, Compaq). I have an old 770X that was in a fire and stepped on or otherwise crushed so as to break off the little feet in the back. It still works reliably and I even fixed the screen with a $35 replacement from e-bay (unfortunately the backlight is still broken
- needs a new inverter I guess).
...and another thing: I have also found that IBM goes to greater lengths than the others to make their machines behave well. Things like custom ThinkPad utilities that actually help you configure and install things instead of just opening the Windows control panels, and various software, firmware and hardware features to improve security, reliability and maintenance. IBM is a company with an actual R&D budget that really DESIGNS their computers instead of just building boxes. (Sony machines are nicely STYLED but seem cheesy to me in actual use.)