Wow, great thread. There are so many bands that have grown on me in the last few years, especially pertaining to an album-to-album growth.
Nocturnal Rites: Definitely one of the forerunners in that respect. I have loved these guys since Tales of Mystery came out around 1998, and with every album there is just amazing growth. One thing I love about these guys is that not one album sounds like the previous, they always forge ahead, yet still stay unique and have their own sound. You have to respect a band like that.
Firewind: While these guys have gone through some shaky lineups, they have always managed, for the most part, to deliver quality US styled power metal. I thought Allegiance was easily one of the best albums of last year, despite what a lot of critics have said about it. Each predecessor album suceeded the one previous, IMO. Yet, their entire catalog to me as a whole is just great, with not one bad album. I do hope Apollo does stick with the band this time around, and they finally have a permanent vocalist.
Vision Divine: Here's another great example of a band that has grown with me. I mean really, look at the progression from their S/T album back in 1999, to what they're putting out now. This band for me has become the new Labyrinth, and I follow these guys with every release. Adding Michele Luppi has just further extended my love for this awesome Italian power metal band. Stream of Consciousness, Perfect Machine, and their newest, 25th Hour are just some of the finest examples of progression in the power metal world.
Pagan's Mind: I think mostly everyone has touched on this band in the previous posts, LOL..but yes this band just keeps getting better. I just ordered God's Equation earlier tonight, and I can't wait to hear how this one sounds.
Threshold: While I don't have this band's entire catalog, what I do have from Critical Mass - Dead Reckoning shows a LOT of growth. I just love how this band gets more technical, yet more accessible with every album. This band is truly destined to be the best prog-metal band in the world, and i'm behind them 100%. As it stands, Dead Reckoning is definitely on my yearly list for one of the top albums this year.
Nightwish - Beautiful example of growth here. I didn't like Angels Fall First much when the buzz started with this band back in 1997, then I checked out Oceanborn about a year later..the rest is history. With every release after Oceanborn, I was completely hooked. I thought my worst fears may have come true when Tarja left this band, but like a lot of people I imagine, we were wrong. I think I love Nightwish more than ever these days. Actually i'll even go on to say that Dark Passion Play is the album of the year for 2007 for me.