i've used all of them bar the full blown thc and nexgen. personal opinion XBMC is what i use predominantly as a front end mainly because i use my xbox more as a media center than a game center (but even for that XBMC is adequate)
EvoX essentially only gets used infrequent but important tasks such as hd locking/unlocking and bios flashing, no offence to the avalaunch team but i'm not enabling eeprom writeback on my machines for the forseeable future.
Much as the fan boys like to create a competition between the three i think unleashx, mxm and avalaunch all have they're good points- i like mxm and unleash's preview vid capabilities, generally their skinning engine is more advanced than avalaunch's (such as completely movable elements unlike ava's, and the use of videos for backgrounds)
I like unleashX text editor feature, and eventually when the game save manager works that'll be another plus point.
obviously this is in the ava forum so i better say something nice about them too
, ava no doubt has the best net engine, irc, remote ftp, telnet, shoutcast, acam/apod, first to introduce downloadable skins and only one to support downloadable game saves- but one thing (and no flames) the actual front end could do with a rejig, there's very little flexibility in the appearance other than the background
tHc lite is very slick but its lack of ftp will always count against it
but yeah if you were asking me to pick one definitive one- for me and my usage- xbmc