Hands down that would be my PowerMac G5 Dual CPU 2.0.
I bought it in June of '05, and I've never had so much as a kernel panic on the beast. And I've done a LOT on that machine over the years. At times I wish I could upgrade the graphics card (AGP 🙁), but other than that the thing is still as fast as ever. Of course anymore the graphics card doesn't bother me too much as my pc gaming days are behind me for the most part. At least until StarCraft II comes out, and then I'll be whining about graphics again. 😉
Second place goes to my mac mini CD 1.66. I bought that in September of '06 and it hasn't given me a single problem either. I no longer use it for my office, so it's been connected to my TV for the last 18 months.