I haven't listened to Black Sabbath in quite a long time. I just realized how awesome of a song "Megalomania" is. What other songs have you had for a long time but didn't realize they were awesome?

Megalomania is amazing ( as all B. S songs) but by the same guys, Sabboth Bloody Sabboth. That song is very likely (now) my favorite.

I had the live album Death on the Road by Iron Maiden for months before I first listened to it on a car ride to New York from Illinois. Everything about it was so good.

Cajun Hell by Exodus. Zetro's voice has the most perfect natural essence of being really fucking pissed off all the time.

That's my all time favorite song of theirs, followed closely by Into the Fucking Void. Sabotage was easily the creative peak of the Ozzy era.

As for stuff it took me a while to appreciate, Primordial springs to mind. Love them now, one of my favorite bands. But it took me three consecutive albums before I really got hooked.