Yay, I get to be the first to say:
BLIND GUARDIAN - Tales from the Twilight World/Somewhere Far Beyond/Imaginations from the Other Side
I love the first two albums, but TftTW is really where they began to hit their unique sound, SFB solidified and improved on it, and of course IftOS is an all-time masterpiece, widely regarded as BG's best (for me it's between this one and Nightfall in Middle-Earth, their sixth album).
Let's look at Katatonia....
Discouraged Ones/Tonight's Decision/Last Fair Deal Gone Down
Obviously DO really began the 'gloom-pop' stage of the band, although I'd say the change really began on Brave Murder Day, at least musically. Much more straightforward than Jhva Elohim Meth through For Funerals To Come.... Like BG, their fourth album, (my personal favorite) TD, developed their sound further, and LFDGD did the same, while experimenting with some new things (more daring vocals, electronics).
Opeth we all know about, Nevermore's been covered.... Now, at the risk of getting flamed:
SOILWORK - A Predator's Portrait/Natural Born Chaos/Figure Number Five
APP, being the third one, is generally regarded as their best, and I agree. Also the first album to feature clean vocals. NBC saw the clean vocals taking on a larger part, and the music taking on a less thrashy sound. FNF is hated by nearly everyone (unfairly), the music being less thrashy still and yet more clean vocals. Also, Speed's screams are a bit weaker. But still, the music is following in the path that APP blazed, for the most part. At least, it certainly is not like the first two albums.
Maybe now for some:
SYMPHONY X - The Divine Wings of Tragedy/Twilight in Olympus/V - The New Mythology Suite
TDWoT needs no introduction. Their third album, widely considered their best. I don't necessarily agree, but it did indicate a marked improvement over the second album, and definitely the first one. TiO is held to be inferior to its predecessor, and maybe it is, but really, it's just different. A damn solid album. Then came V, which is godlike, easily their best, IMO. It is one of the only albums I've ever heard that when I listen to it, I think, 'There is nothing at all that I would change about this album.' Fucking amazing.
Iron Maiden's already been discussed, so no need for it here.
But I need to mention:
VINTERSORG - Cosmic Genesis/Visions from the Spiral Generator/The Focusing Blur
Of course we know that Cosmic Genesis began his move away from folk metal and into space/prog metal. Visions took it even further, with weird songs like ESP Mirage. Visions is underrated sorely, IMO. TFB is WAY out there, and brilliant, still on the space/prog metal trail.
Well, I'll end here. My 3-4-5 list was not based on what I think are a band's 'best' albums, just what I think to be the most defining.
Ryan