Isis' sounds definitely changed a lot throughout their career. Earlier stuff was pretty sludgy and doomy and later was post-metal, but I dug it all for the most part. My favorite album of theirs is Celestial, it was 2 releases before Panopticon and it falls right in the middle of sludge and post (though I guess they can be considered one in the same). Oceanic is brilliant too.
It's too bad you weren't into them live, Sneaky. I saw them once and the show was amazing, though my cousin who wasn't into them that much also didn't like it either. Thankfully when I did see them it was a 20 year anniversary tour and they played something from every album instead of their normal play most of the latest release and 2 older songs. Jesu is great though, I definitely have to catch them some time. I have not checked out Twilight, but I definitely will.
For more post-metal, check out Godflesh, Jesu, Neurosis and Pelican.