Personally, the morrow archon feels like the most powerful of the archons. possibly one of the better looking arts, too. works really well in the theme, plays like a solo that actually has mobility and abilities. they all do to some degree...i think they only the the protectorate one is really just another piece in an already crowded table.
runners up- void archon. also an extremely scary archon, has the most mobility and some of the best assasination tricks, the art is good....and it seems like a cool fluff fit for skorne.