My top five favorites:
Jim Sterling (THANK GOD FOR HIM)
Stryder7x (does a bunch of really cool glitch videos about Paper Mario)
FamilyJules7X (his guitar covers are amazing)
Mark Brown (also the maker of Gamemaker's Toolkit, he does a lot of fascinating game design analysis videos, including a series called Boss Keys that analyzes the dungeon design of each of the mainline Zelda games)
AlphaOmegaSin (one of my favorite youtubers, he may be controversial a bit at times, but he's pretty awesome)