I can't tell who's being sarcastic in this thread. Daedric and Nightingale are prolly the two most popular sets for players, taking into consideration both aesthetics and stats. Least popular...hmm...tempted to say Dragon. Takes an extra perk AND is slightly weaker than Daedric.
Elven Armors because they look so stupid, also F the Thalmor.
At least that set doesn't look as stupid as it did in Oblivion. Especially those goofy looking "dragon foot" boots they had in Oblivion. They looked absolutely ridiculous! The Dwemer boots in Skyrim aren't much better. They also look absolutely ridiculous. And I have no idea at all as to why the Falmer boots are actually called boots at all.
I'd say all ugly armors/armor pieces, especially the ugly helmets (leather, scaled) and rightly so.
There are too many players out there wearing ugly daedric armor thinking their 1000+ armor rating would grant them any more advantage compared to armor sets hitting the 567 armor cap.
With my vamp mage I just aquired the Falmer set for Mjoll. Looks wicked on her on my main toon I use full daedric since he is a demon/daedra master and so happened to conjure up his armor below the College
Least used might be uhmm low grade leather and iron on players and ebony and stuff on npcs.
Glass - I put it on once and couldn't believe how ugly it is, on the flip side the glass weapons look great but damn the armour is some awful looking equipment. Dwarven, orchish and dragonbone are awful too for me.
I have only ever found an npc wearing ancient nord armour once. This wasn't in the ancient nord ruin where everyone is wearing it. It was worn by a bandit.
First armor i always take(after getting out of Helgen) is to obtain a whiterun Guard armor set But most underused? I say the guards' uniform, especially the ones like dawnstar