All of Ihsahn's albums are worth listening to a lot. I'd have to say AngL is the weakest, though it's an extremely solid album. It's memorable, but it pretty much just sounds like Emperor with some catchy riffs. After, The Adversary, Eremita and Das Seelenbrechen are where it's at.
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Das Seelenbrechen holds up to Ihsahns other releases. Sure its no Adversary, After or Eremita but it shows that he is expanding his sound and staying interesting at the same time. The album isn't as structured as the last 4 albums, but I'm excited to hear how that sound will be perfected with the next album.
Best place to start?
Prometheus is fantastic, as TheNightsnail recommended. However, in my opinion the first solo album "The Adversary" is a better place to start as its a perfect balance between Emperor and what he would go on to do with the other solo releases. Prometheus is fantastic but uses the low end of that 7 string guitar much more than any other of his releases, so I'm not sure if its the best introduction.
Emperor's Prometheus which is practically Ihsahn's solo album. After it continue to his later solo works. Also, if you're interested in experimentation listen Thou Shalt Suffer's Somnium which is a mainly instrumental release.