I got to say the original Xbox only because I've never come across one that just plain works 1000% the one I currently use won't play certain games (like Crimson skies, Metal slug 5, Still Life) and the other one has a damaged harddrive so it won't boot up.
Interesting... I have 4. 1 I picked up at a yard sale the other two I got at a thrift store. The other I've had for over 10 years. The laser died on the DVD drive but I picked up a refurb from Stone Age Gamer (before I started finding them all over the place for cheap).
They run cool. Are mostly quiet. Built like tanks and after soft modding they are better than an ouya.
Little known fact. Many games actually played in 720p with Dolby 5.1 surround if you used the HD AV kit.
I love the original Xbox. The harddrive is the weakness for sure. But it's easy to dump the info necessary to replace it with a spare later on down the road. You just need to remember to do it before the original one dies.
I agree with the Tiger hate here though. Those games were beyond worthless.