I would have to say that Lufthansa is VERY VERY clean inside and out and I an not just saying this because I am a HUGE fan of Lufthansa. Also a smaller european airline that is very clean but nobody knows about this airline really is Croatia Airlines. They have very clean planes inside and out.
I've never been anywhere near an MEA aircraft, but since they have world's youngest fleet (according to an earlier thread), does this also mean that their entire fleet is spotless? Just curious... i bet it won't be easy to keep those mostly white planes in top condition all the time.
Continental planes are usually somewhat clean, and the interiors are always spotless. On the outside though, many CO planes have absolutely filthy wings, and the actual fuselogue can be somewhat scrawny as well...
USAirways planes are always sparkling, inside and out.