Xbox of course!!
With a modded Xbox, you can play NES, SMS, Game Gear, Lynx, GB/GBC/GBA, Turbographx/PC Engine, Genesis/MegaDrive, SNES, Neo Geo, and a very current port of Mame (v.84 is the latest version I have though I haven't checked for updates lately), with 99% of the games for all of them running perfectly at full speed with perfect sound. N64 works very well as well with probably 50-75% of games working, also mostly full speed and all with sound, PSX works well as well. And I'm not even bothering mentioning all the older (than NES) consoles that it does perfectly. Not to mention ported games like full Doom (of course).
Considering you can pick up a used Xbox for somewhere around $100 and softmod it for free (or chip it yourself from $30-75), I wouldn't say it compares to using even a cheap PC to emulate stuff. Also, you know, I'm sure all of us would much rather sit on our comfy couches playing emu's thru our Xbox's on nice large TV's, rather than sitting at a damn computer desk to play them all day. And don't pay the $250 for a premod... it's a ripoff. IMO, chipping or softmodding are both super easy. Hell, the chips they sell these days don't even need to be soldered in, they are litterally 'plug and play'. And there is so much documentation on the net on how to do any of the methods availabe that there's really no reason even the most technically uninclined person couldn't do this themselves.
Unless you're talking NES and earlier, there's no way in hell you can compare emu's on the DC to emu's on the Xbox. 'Nuff said.
And I've softmodded mine and it works great, I love it and played the hell out of it. For anyone who doesn't know, softmodding is the process of hacking into the Xbox using software only and setting it up to run just like a system modded with a chip. That's right... you dont' even have to open it up to enjoy all this stuff. It's not too hard to do, you gotta read all the readme's VERY carefully and take your time with every step, it is very easy to mess up if you're not being careful, but even if you make a mistake, MOST mistakes are easy to recover from. I cannot stress enough though, read EVERY WORD of the readme's, FAQ's and tutorials, then read them again. I hate to say this but don't expect a lot of help from anyone if you get into trouble, as most xbox sceners turn up their noses to guys who do softmods and can be real jerks. Anyone interested in doing any of this should check out xbox-scene.com. Actually I take that back partially... the guys on the xbox scene forums are pretty helpful as long as stuff you're asking isn't covered in the tutorials, but forget talking about softmods on the xbox scene channel on IRC
Also, the xbox emu/homebrew scene is HUGE.
