I guess it depends on "best for what". As TOP mentioned, if you're looking for best performance, there are specific things you should look for. If you're looking for portability (weight, size), you aren't going to get something that's blazing fast. Battery life also is at the expense of performance.
I got a Hypersonic Aviator FX7 with an AMD 64 X2 4800+, and it rivals nice desktop machines for SolidWorks rebuild times. But there are draw backs, such as:
- very heavy
- 45 minute or less battery life
- gets pretty warm
- fans (4 of em) can be loud
There are several manufacturers that make similar systems, as someone else has mentioned. Probably the most popular is the Alienware, but when I bought this a few months ago, the Hypersonic was the least expensive - $3300 with a small but fast hard drive (90 Gb 7200 rpm SATA), no bluetooth, but a webcam and mic, no built in wireless, using a pcmcia, 2 Gb RAM, 1 DVD drive, Quadro Go FX 1400, 17" 1920x1200 display (very nice), full size keyboard including num pad (I hate having a computer without a num pad). If you go nuts with the options you could easily spec a $5000 laptop. Because of the weight, this is not something you look sexy with at a coffee shop, but it is easily something that can completely replace a desktop, and you'd never miss a step in the speed.
Dell is of course the safe choice. I've had other Dells and they work fine, but this goes the extra step.