So there is already a poll about the biggest disappointment in metal history, but what about those albums that have been surprisingly good? Once in a blue moon one of our favorite or not so favorite bands releases an album that exceeds our expectations (often after a long period of creative bankruptcy), so what albums have done that for you? Please discuss (or just name them for others to discuss if you're in a hurry).
The biggest leap forward in quality in my opinion was the second album of one of my favorite bands, namely "Oceanborn by Nightwish. It was and still is a true classic of the symphonic metal genre, but it was preceded by Angels fall first, which suffered from pretty poor production, some very awful lyrics ("Nymphomaniac Fantasia") and a less than enjoyable vocal performance by Tuomas Holopainen on two of the songs. The songwriting was also very far from what the band was actually capable of or became capable of with their follow-up album. Oceanborn was a masterpiece that I don't think anyone could have expected based on the band's previous album. So that's my biggest surprise, what is (are) yours?

Painkiller by Judas Priest came after two bad albums and became Priest's opus.

@Ernil strange, i actually find Nymphomaniac Fantasia to be one of the most criminally underrated Nightwish songs, and it's really raw (relatively speaking of course, we're not talking Hellhammer here) for a NW song. Even now i can't stand their latest more emotional (read: kinda cheesy) works, but i still enjoy that one, it's really gritty, especially in the lyrics department, if it referenced vampires, a person named "Elizabeth" or female demon-nymphs it might sound like something Dani Filth would write. The rest of AFF isn't that bad imo either. There's a difference between that and Oceanborn, sure, but i don't see the change in quality to be as big.

i was surprised how good Blood of the Nations by Accept actually was. The first two tracks that were previously released were already awesome, sure, but I didn't really expect it to be so consistent and awesome. Definitely my album of the year in 2010 and silenced pretty much all of the Tornillo-haters. Definitely the most epic metal comeback in recent times.

Another one would probably be, ironically, UDO's last album. Sure it's nothing original, but it's got that modern relentless metal fury that on Dominator kinda feels a bit held back, while the songs Rev-Raptor and Renegade are total breakneck overkill.

Septic Flesh - Communion
cradle of filth - Godspeed on the Devil's Thunder
Trans-Siberian Orchestra - night castle

Van Halen's new album is currently among my favorites this year, and I was expecting it to be complete crap, seeing as I don't care much for Van Halen in general, thought that the vocals would be awful due to age, and the awful initial impression that Tattoo left me with. Turns out it's a fine collection of old tunes, with a lot more speed than I thought it would have.

Agree on Nightwish. They also gave a huge leap forward from DPP to Imaginaerum, though not many people seem to agree with me

Evanescence's latest record also surprised me a lot with a huge band oriented album, that is what the band was missing to come full circle in my opinion.

I guess Pain Of Salvation also surprised me with their 70's approach on their latest albums, that was a refreshing change.