DEFINITELY Daud.
For the reasons that Corvo is basically an empty shell, and for the low chaos playthroughs...
I don't know. I thought that being a low chaos Corvo would be nicer, but when I was at the last level as Daud... well, it's a bit of a...
Spoiler
When I was freeing the assassins from the Overseers, it struck me how close I got to them. They felt like innocents that I needed to save. Once, I was busy taking out two of the Overseers, placing them on a roof so the rats can't get to them, then I returned to the assassin to rescue him, then I found that there was just one Overseer left, and he had a grenade out. Right before it seemed like he would throw it, I got him in a chokehold and saved the assassin.
I felt like a savior for the assassins, and I never felt like that with Corvo...
...and when I played as Daud in the high chaos route? I felt way more awesome then Corvo, because every time I planned a big kill, I thought to myself, 'What would Daud, the killer of an Empress, do?'
Not to mention his magic sight thingy was way better then the heart, to me, and his Blink was way cooler for me, and the double jump was more like just a prolonged single jump, which felt more realistic, if a little floaty. XD
But, I like Corvo a lot too. I guess if he had more vocals and if he had more of a backround, then I would like him better. I just felt that Daud was cooler, more bold, and as we all know, the boldest measures are the
safest coolest.
Just my 2 cents.