I voted for XBox, but each system has it's own good points...
The PS2, as has been mentioned, is backward compatable. That combined with the large number of RPGs for it, make it a good system. However the PS2 has the worst rating of the three for long life. The local EB guys don't recommend you buy a PS2 without the extended waranty. The PS2 is also the worst system of the 3 as far as power, when it came out it blew everyone away, but it's just not as powerful as either of the other systems (though I know that's not always the most important thing, as we have plenty of people on this forum who play both the N64 version of Starfox as well as the original SNES version).
The Cube is a killer system. Like most of big-N's systems, it's sturdy. You know it's got to be sturdy if they've bothered to put a handle on it so you can carry it around (anyone lift an Xbox... don't hurt yourself). The Cube has some of the best specs for good looking games too (they can just make prettier ones). The down side(s) to the Cube include a lack of number of games (PS2 will always top that list, but even for good ones, the Cube is still low on the list) and the lack of a hard disk. There are however several games on the Cube that will cause me to pick one up at some point.
The Xbox, dispite my extreme dislike of Billy-Bob Gates and M$, was my choice for a Next-Gen system.
The Box is a souped-up PC which has a couple of things going for it, namely that lots of the new games comming out on the PC, will also appear on the Box (Deus Ex, Theif 3, Morrowind, Splinter Cell). It also means that (at least at the time it came out) it kicks the butt of most home PCs for power (and does a good job at kicking the other Console's abilities).
The Box has a hard disk... no more hunting for that stupid memory cartridge, or worse, finding out you're a few blocks short and have to decide what to remove from it to fit the new game on (can't tell you how many times I had to do that with the N64). Granted, the newer systems have much bigger mem-carts, but still....
One Word..... Rare (silly Nintendo).
One last thing that makes the XBox great.... if someone breaks into your house, you can beat the stuffing out of them with it (don't try that with your PS2 or Cube!).
PHIL