Today's a special entry in WIYF, because it covers a category to broad and beloved that one simply can't be limited to one answer. I'm talking, of course, about music albums. The only rule is that you can only list one album per band.
Music is like religion in a way, because most people's musical tastes are so focused and diverse from the tastes of others that a person could, reasonably, own a CD collection that encompasses just one obscure subgenre, like Polish folk music, Finnish death metal or gothic djent. For me personally, I'm a sucker for metal - mostly prog metal, but also power metal and thrash metal; any combination of those three is simply sublime. But even in a genre as narrow as progressive metal, I still hate the sound of bands like Opeth, Tool and Porcupine Tree. But some people love it, and would label bands like Dream Theater, Symphony X and Ayreon as too light and fluffy.
My Top 5 albums, in absolutely no order (because that would be damned impossible) are:
Ayreon - Universal Migrator
Symphony X - V
Dream Theater - Metropolis Pt. 2: Scenes from a Memory
Vektor - Outer Isolation
Ilium - Ageless Decay
[Honourable Mentions: Megadeath - Rust in Peace, Sonata Arctica - Reckoning Night, Heavenly - Dust to Dust, Death Angel - The Ultra Violence]