Okay, I'm going to make a list here. I just take a couple of old bands I listen to and list their most rocking songs (with rock as in the genre).
And why a list you might ask when he asked for the best rock song? Because I'm an indecisive prick, that's why.
Highway to Hell- AC/DC; A lot of ACDC songs are applicable for this list, but I chose this one. It just has the best riff. Thunderstruck is a good second here. http://www.youtube.com/watch?v=erJc4dzZ3IA
Black Night- Deep Purple; another great band, and even though they've made better songs (listen to their 'Made in Japan' album), it's the most rocking one, imho. http://www.youtube.com/watch?v=QHlODWd4GeM
Locomotive Breath- Jethro Tull; A very classic intro, but a timeless riff with good lyrics and an awesome solo. Minstrel in the Gallery was another contender for this slot. http://www.youtube.com/watch?v=RO8aa9PGxvU
Whitewater- Kyuss; Okay, some people might say that this is more metal than it is rock. However, I think that this is one hell of a rock song. http://www.youtube.com/watch?v=KcY3UF6_IaM
Rainbow Demon- Uriah Heep; this is one of those old singalongs. I mean, come on, who has heard this songs and didn't want to shout "Raaaaiiinboooooooow Deeeeheeemoooooon" at the top of his voice? http://www.youtube.com/watch?v=Eku0Pg3c5_c&feature=related
Won't Get Fooled Again- the Who- I saw the Who mentioned in this thread with their song 'Behind Blue Eyes' but I think this one deserves the title rock song a lot more than that one does. http://www.youtube.com/watch?v=sO8GhA8OdyQ&feature=related
[insert song]- ZZ Top; Really, I wanted to say 'Give Me All Your Loving', but I couldn't do that with a straight face. All of their songs apply for this list.
EDIT: Fuck, apparently I can't read. I thought he asked for the most rocking songs.