Definitely didn't forget about this wiki for a while...heheheh...
MY Top 8 HARDEST levels (get it? WeegeeSPIDER, Top 8? Hahahaha-sorry) :
All of these are judged by the fact that you start as the kid, whilst most videos listed do not follow this rule, it is still rather easy to tell how difficult it would be had you started with him. Also, all of these videos are not recorded by me and all credit goes to their creators.
https://youtu.be/WbW5EPWUo4M (Final Marathon)
https://youtu.be/bFWYM9ojqZ0 (Alien Isle)
https://youtu.be/u5joaYXCsQQ (Alien Twilight)
https://youtu.be/ziXz9s_AVgg (Monster Island)
https://youtu.be/6_lzBlVr8m4 (Ice God’s Vengeance)
https://youtu.be/aerRQBkDjNk (Forced Entry)
https://youtu.be/WEq-a6rJa1E (The Forbidden Tomb)
https://youtu.be/Lh7xN3lRkgk (Bloody Swamp)
Bonus! Top 3 hardest Elsewhere Levels:
1: https://youtu.be/EQsxH4m33Lk (28)
2: https://youtu.be/y-5SKGEeRIs (16)
3: https://youtu.be/VVJ5-mZQfSc (15)