The first Assassins Creed is rather boring and repetative.
AC2 was excellent, with more variety, new weapons etc. and had a interesting story. Brotherhood continues right from where AC2 finished, and is set in Rome, plus you can ride your horse through the streets which is pretty cool, but on my PS3 it has a hard time keeping up with loading Rome.
I've not played Revelations yet, nor AC3, but i've heard great things about AC3!
EDIT: Just thought i'd add, i've platinum'd AC2 nearly twice over, and got around 70% of the trophies on Brotherhood. (AC1 doesn't have achievements on PS3)