Damn hard choice, so many songs have huge riffs. I'll go with Symphony of Destruction by Megadeth, with honorable mentions to Seek & Destroy, Back in Black, Raining Blood and Iron Man. Symphony is just so perfect, so evil and cheering at the same time.
Tu-ta-na ta-na-na... Ta-na-na Tu-ta-na ta-na-na!
By the way, people really give weird examples here, for songs that aren't really riff based songs (at least for me). Stairway to Heaven? Really? It's (obviously) not a bad song but the riffs aren't the main attraction there, it's the enigmaticness of it and the solo, or is it just me?