Oh, it's so hard to pick a favourite... I'd say I'd have to go with sister location gameplay-wise and fnaf 3 lore-wise. I like the variety in gameplay of sister location where every night is different. in fnaf 3, the mini-games bring a lot to the table and if the story was wrapped up there I would have been satisfied.
i always loved fnaf 2. the songs people made for it were so good and its by far the most nostalgic for me. also most of my fav characters came from that game
although in terms of atmosphere, it kinda sucks... fnaf 4 always got me in that sense