I can't believe I didn't vote in this by now.
Glass. Why ? People often talk and talk and then talk some more about joys of Ebony or Daedric armor, about how much protection these overly heavy pieces of armor offer... hello ? The best AR/Weight ratio of the game belongs to glass. The ONE and ONLY elite unit in the game [put in there just to annoy those Indoril rednecks and make them feel like they should - INFERIOR] utilises Glass. And finally, Glass armor pieces are much easier to obtain within the game, unlike Ebony or Daedric. So suck on that, Mr. Fyr !

edit: Of course, I am only taking original MW into concideration, without expansions and mods. That discards High Ordinator armor which is interesting but still too damn heavy for my taste (that, and pauldrons look hillarious

)