@twinsun: You could run on rooftops and stab things like a ninja! Just climbing things was fun! All the repetitve parts took at most 20 minutes or so out of a very unique experience in a unique locale with totally alright combat!
Now it doesn't hold up worth shit. Especially not compared to Brotherhood, but please. Have some mercy on the origins of the series.
Well, fair enough.. But still, if you are asking which one I find the least enjoyable, I would still have to say the first, even if it at the time did some cool stuff.