Vagineer - the Engineer with a faceposer'd face that makes his mouth look like a vagina. Tends to be insanely loud and speaks backwards. Don't feed him.
THIS VIDEO CONTAINS _______ - Some Kirby song with the usual 'bonk', 'pingas', 'toast' etc pasted over the top in the same pitch as the notes. Can contain other songs :3
SCOUT IS DELICIOUS - This is the big one. Hell, this is probably the biggest fad on YouTube. It has over 3,000 responses for god's sake. If you don't know it by now you have alot to catch up on.
Painis Cupcake - The Soldier is the Painis Cupcake, he'll eat you. End of story. Has sex with Vagineer occasionally.
Kau - Some Russian song or something. I dunno.
Bonk songs - Yeah, it's songs with bonk.
______Cube - The GameCube logo with other stuff over it.
This is illegal you know - A line from the old CD-i Zelda game.
Pingas - Another big one. In the old Sonic cartoons, Robotnik says "Snooping as usual, I see". If you clip out the "ping as" it sounds like he's saying penis. It went viral instantly xD
Trolololol - A fad spawned from a Russian guy singing a song where it sounds like he's saying "lolololol, lololol, lololoooo, lolololoooooooo~". I use this on the Escapist a fair bit.
NO - A line from Hotel Mario.
lol3links
___________ in a nutshell - I love these, they're videos pointing out massive flaws in the design of TF2 maps, and showing what sort of uberspam happens practically every round. Sometimes inaccurate, but they always crack me up xD
Strange videos featuring Engineer - Name says it all.
That's all I'll post for now. Obviously there's shitloads more out there, but these are some good fads to get you newfags into YouTube

And now some of my personal YT favourites.