Because the first one evoked a feeling that the other games cannot even come close to matching. There will never match the visceral feeling that you got when Garnier de Naplouse had the ill man's legs broken right in front of your eyes, and you and everyone else was helpless to do anything to prevent it from happening.
That, and most of the other cinematic scenes in the game is what clenched that top spot.
The other games were perhaps more polished as games, but I felt they rushed to appeal to the populace's desire of more variety.
"What is best in life? To crush your enemies, see them driven before you, and to hear the lamentation of the women. That is good!"