evox was everyones first dash, it was out a long time before the other dashes really, and was used in all the early installers as jaydee said. thats really the only thing it should be though, even the things it does well, other dashes do way better now.
mxm was the king for awhile as far as I'm concerned, it was a sweet dash with a very nice community behind it, and a very dedicated single developer. but sadly, the inherent fault in one dev is a no fault tolerance, and as development waned, mxm faded into obscurity. this wasn't helped by the fact that as it slowly lost momentum, unleashx came out of nowhere.
unleashx grabbed the best of all the dashes (quick menus, video backgrounds and icons, gamesave downloads, avalaunch boost mode, xml customization, mp3 playing) and served as my main dash for most of all of my xboxes lives.
but, as I began to play with media more, and I had more places to watch it, and once I started recording with Windows MCE, it made no sense to use anything but XBMC.
Theres a few things I miss about unleashx enough to keep it around as an app forever, but no other app, dashboard or otherwise, can compare to the sheer breadth of features XBMC has.
Other dashes and ms dashboard replacements will come and go, but unless they offer something very radically new, for me there will be no reason to run anything other than an XBMC dash from now on. But if you're in this forum, you can more than likely assume that will be the eventual case for everyone.
So yeah, short version, XBMC.