Lessee... I got about 600 hours worth and I'm not even close to really stop playing it. Oblivion is... a vice.
The Debaser, Argarmir's little blade, is unique in appearance and power for some reason. It has the reach and speed of a short sword but has the weight of a long sword (based on level I would think). Its an oddity that always got to me; and it looks pretty cool too.
Another odd weapon would be the Truncheon of Submission: It damages their fatigue massively, but it heals them far more than it damages.
It really is a pain to choose which weapon I like better: The Sword of the Crusader (Knights of the Nine), Dawnfang/Duskfang (Shivering Isles), or Umbra (Oblivion). If I'm blunt it's the Mace of the Crusader without argument... And if it's marksman I have to make my own bow otherwise suffer pain trying to find a good one in game.