No riffs are mandatory to know you idiot. Guitar (well, music in general for that matter) is about playing what you want. Not playing some tired old classic rock riff that everyone is sick to death of.

Ibanez RG1570 Prestige
Laney VC15
Boss DS-1
Jim Dunlop Crybaby
Quote by Daisy_Ramirez_
As above, what songs/riffs is it mandatory to know?

Fermented Offal Discharge. This thread sucks.
Meister der Marionetten
Dear God, do you actually answer prayers?

Yes, but only in a way indistinguishable from random luck or the result of your own efforts.
enter sandman, iron man, smoke on the water
Gear: ESP MH250 NT
ESP EX400 ow
Ashton Dualler

Marshall MG50DFX

Jim Dunlop Cry Baby Wah
EHX metal muff
Quote by sashki
There aren't any MANDATORY riffs, but there are riffs everyone knows anyway. Mainly Smoke on Teh Water.

Everyone plays it incorrectly though.
Iron man, enter sandman, back in black, smoke on the water, fat lip.
My Gear:
Gibson Les Paul Studio
Dean Razorback Two Tone
Ibanez 7 string
Randall RH150G3 Head and Cab
And lets not forget Crazy Train and Dazed and Confused.
Quote by LochieW
Thats it, attackofnose wins LochieW's Official Trophy of Using His Googling Skills to the Max!