I will not rate them cos they're so different, but here's a few comments on each of 'em:
Vargnatt - simply amazing. lots of variation. very fierce vocals and some clean high vocals. the acoustic piece "Trollskogen" is great. they show that they were skilled even back then.
Bergtatt - another great one. i think the production is a little tame for being a black metal album though. could have been more violent
Kveldssanger (not janger or fanger) - very nice when you want to pace down from all the BM pounding in your stereo. strong, haunting vocals
Nattens Madrigal - a shock at first, but then after a while you realize how great this is. a little bit more variety of speed would make it a true masterpiece, even though the production is (deliberately) bad
Marriage of Heaven & Hell - Also great but no more metal. Good production and very strong singing. nice acoustic parts as well. I think they could have skipped some of the fillers of "industrial noise" which aren't even songs and fit it all into one disc
Metamorphosis - hmmm, the last part of Gnosis i really like, but the rest...i would never listen to it if it wasn't Ulver
Perdition City - Again, a very good album. they made a complete album in the same direction as the parts from Metamorphosis that were good
Silence teaches.. - totally uninteresting if you ask me.
Silencing the singing - as i said before about metamorphosis, nothing i would listen to if it wasn't Ulver, but it's ok. it has its moments
Lycantropen - sounds nice but a bit boring, same thing through and through
1st Decade - just crap. this isn't even an Ulver record and you can hardly hear anything that relates to the original songs. i won't even try to get into this
A quick Fix of Melancholy and Svidd neger have i only heard some samples from so i think it's too early to say
That's my opinion