to limit yourself to one genre is pretty ridiculous - but rock & roll has become a term that defines an all-around basis of pop music in the 20th century, so:

top five greatest:
1.) the beatles
2.) radiohead
3.) bob dylan
4.) the rolling stones
5.) the velvet underground

top five favorites:
1.) nirvana
2.) the beatles
3.) neutral milk hotel
4.) radiohead
5.) the smiths


the power of accurate observation is commonly called cynicism by those who have not got it. george bernard shaw