XBMC - Use a dedicated HTPC box and run the linux/windows version of XBMC.
PLEX - Use a Mac Mini.
If you want the option to run XBMC and PLEX on the same box then use a Mac Mini. If you only want XBMC then a PC based box is much cheaper.
The ATV2 will run XBMC and a stripped down PLEX client but you asked for the best hardware, and while it's the cheapest option, it's not the best hardware.