Im starting a new job that involves me tavelling ALOT of the time, so im thinking of investing in a decent mp4 player to make the long trips less tedious, but there are so many i dont know where to start - which one is best?
If you're wanting it for video and aren't bothered about a really small device, check out the Archos players as rojash says -- they're better at video and some other things than the iPod video.
Else, if you want something very small and pocketable, and can put up with the smaller screen, have a good look at the iPod.
If you're mainly also wanting it for music, also consider strongly the iPod, as it's better for this use than Archos players (in terms of ease of use).