I don't know what their best album is, it's probably Once Sent From The Golden Hall, but my favourite is Versus The World. Tis the most epic and brooding, with the best melodies, although the generally slower pace throughout the album can make it feel a little samey. Still, ...And Soon The World Will Cease To Be is my second favourite metal song ever, so I have to love it...baby!
I don't know about them really. I suppose every song, and every release taken on its own is wonderful. They're a strong, solid, and distinctive band. But....considering they do the exact same thing on every album, well, it gets a little old to be honest. Comparable bands are Bolt Thrower, Malevolent Creation, Cannibal Corpse....bands with lots of releases, and doing the same thing over and over again. Not inherently bad, if the music is great, as it is in all the bands I mentioned I think, but it starts to become unsatisfying. Amon Amarth are pretty bad for sticking to a formula, however successful it is. They're going to start running out of melodies in the next album or two, I guarantee it. Fate of Norns is slightly subpar with the melody I think...