I can safely say that fnaf 1 is the easiest game. you have 4 animatronics to be aware of, bonnie and chica being the door stalkers while foxy and freddy rely on watching cams which is an easy task
Next up should be fnaf sl. ive barely played fnaf sl tbh but its all easy and not that complex or scary because it revolves more around a story
Fnaf 4 on popular contrast isnt even hard, its reliant on sounds. its rather the creepiness level that makes it harder for players to play because they panic. all you have to do is to check 2 doors and a closet, the back of your bed and have a good ear
Pizzeria simulator is some different topic, there you have to learn some mechanics in order to beat the game. i think the nights are also longer and you have to pay attention to vents
Fnaf 3 is next. it may not look that hard due to the fact that you have one animatronic that can kill you, that being springtrap. if youre good enough and be careful to avoid the phantoms, you can easily lure him into areas away from your office but it requires good strategy because those systems fail. keeping the sound device working is very important
Fnaf 2 is probably the hardest here. it relies on many things to do in order to not fail, such as winding the music box, checking the hall and the vents as well as having many animatronics. you also have to wear freddy's mask to prevent from being seen by animatronics. that took me quite a while to learn how to avoid their attacks because each of them have different waiting patterns and even if they disappear from your view they may be in your blind spots and eventually kill you once you take it off