AC2 . Goes into good backstory and really fleshes out who Ezio and Desmond.
My ranking from best to worst (although, I like them all)
AC2, 4, 3, AC1, Brotherhood, Revelations, , Origins, Odyssey, Syndicate, Unity, Valhalla.
I haven't played the new one yet so can't rank that. Also PSP AC Bloodlines was awesome for a portable ac and vita's liberation was different but still fun.
Get Ezio collection it has AC2, brotherhood and revelations. oh wait , pc doesn't have the collection (so weird), I would still say AC2 , as it limits your area , but expands gradually and has a real plot. AC4 does not and is massive.