I'll say khajiit, if only to stay consistent
I've played a khajiit assassin with mostly same skills, that worked very well.
For some reason people here recommend Argonians as stealthy characters, maybe one day i'll see if that works too.
I found its a cross between roleplay, some good starting skills spread out, and many of the abilities are permenant...
That if playing vanilla would require constant recasting of spells, or the use and juggling of enchanted gear that could be better use elsewhere.
This was my reason with the added effect that a high level stealthy argonian / vampire hybrid is imo the toughest character to with stand many annoying combat effects..
( poisons, diseases and paralysis )
This however only lasts untill you grow bored and wish to put more role in the play aspect of Oblivion, that and you find out about the Ohmes-raht via mods.