I like Audiopad (Channel) and Teknoaxe (Channel). I don't often use music, so I don't need to search so often, but most of the time, it is just through Youtube in the related videos, just strolling through everything 'Royalty Free' (most of the time these are also free in general).
So I would rate YouTube itself:
Size: 10/10
Diversity: 10/10
Fresh content: 9/10
Navigation ease: 1/10
Quality: 8-9/10 (based on the good ones you find)