I think the original is the best, but I like all of them to an extent. I'll break it down this way:

FNAF1:

PROS: By far the best sound design and atmosphere. Aside from the purposely cartoonish elements like the bank vault worthy panic room security doors, the sound alone makes it feel that you are watching a Chuck E. Cheese-like place during a night shift. Things properly echo, you hear attractions play random music, the animatroincs make sounds that you can tell would be perfectly normal during operational hours, but are intimidating and scary in your predicament.

CONS: On high level game play the game just relies on you doing a very specific pattern and only watching one camera feed or two in order to pass. That alone is a challenge of itself, but I kind of wish the game made you look at more camera feeds and fend off different patterns/conditions in order to make the hard stuff less repetative.


FNAF2:

PROS: Love the designs of all the animatronics, and how the withered original cast has the player believe the story moved forward until the twist is revealed that you are moving backwards. There is plenty to see and do while the gameplay turns what you were used to with FNAF1 on its head.

CONS: While there are more things to defend yourself against, the fact that you have to wind the music box makes looking at any other camera completely useless. As long as you scan the room quickly and efficiently the player can use obvious visual cues and audio cues to win. It's not easy on high level gameplay, but I almost feel Scott should have done away with cameras with this entry since the winding of the puppet was basically all you had to worry about, thus, the only camera feed that mattered as the game got harder.


FNAF3:

PROS: Took the idea of less is more, and I felt he actually did a good job on expanding the lore while wrapping things up. I also liked the hidden secrets you had to do to earn the good ending. It was like a game within a game. Balancing yourself between combating the hallucinations and Springtrap was also very fun and tense in moments where you had to reboot the system(s) and hope you could lure Spring Trap the hell away from your area.

CONS: Like FNAF1, the game ultimately relied on using a room or two through the camera feed in order to win on high level gameplay. This is the only FNAF I did 100% and it probably will only ever be the only one that I do that with. The less is more approach did work, but I felt this game leaned too heavy on expanding the lore (despite how much I liked it).


FNAF4:

PROS: Finally, EVERYTHING you know is different. The game's design is only related to the others in spirit, and I enjoyed how there is more camera movement in general. Also, I love how the game relies far more on audio. Scott's Nightmare designs could have been perceived as edgy for edginess sake, but the premise of the game made those designs make sense, especially once the player knows the story. Oh, and the jumpscare animations are the best of this one, even if one did make me laugh rather than frightened. I also like how the game relies less on speed and that there is NO resource management. Just listen, look, and take calculated risks to win!

CONS: Even before booting up the game I knew that the lore would venture back to the original restaurant. I find myself interested in the story, but again, I really thought the lore/story wrapped up nicely for the third game. It's cool to see the past more closely examined, but I really, REALLY hope Scott means it when he says enough is enough and this is the last game. From my understanding, there is DLC coming to this game, so we'll see what happens then. Also, I do feel the game gets a bit unfair at times and sound cues kind of get lost in the ambiance, so the player ends up losing because of miscommunication, not because of his or her own fault.