I learned Forest of October very early into my playing... I don't really understand what parts are challenging... minus the solos, which with a bit of work also open up.
I'd recommend Patterns in the Ivy. It's a very calming track that won't piss you off as you repeatedly play through it. It's great for building finger-picking skill.
Most of the older stuff really... Advent, The Twilight is my Robe. Probably don't want to be touching too many BWP tracks, because at that point opeth got pretty intricate with their riffs. Still Life you want to stay the living hell away from too, since it has some of their hardest stuff.
My Arms, Your Hearse has some easy tracks. April Ethereal, Credence, Demon of the Fall.
I've heard that Ghost Reveries is overall fairly easy, but I haven't really attempted learning any of it.
Best of luck. Remember, when it gets hard, don't give up.. just try harder.