Wow.. my first post on this thing.
Here's an overview of the Death albums (at least the ones i care to mention)
Leprosy - Primitive but cool.. wouldn't recommend it though
Spiritual Healing - Meh, it's good, but not compared to Death's other stuff.
Human - Heaviest album, I wouldn't say the proggiest. Pretty catchy stuff though.
Individual Thought Patterns - My favorite, I think Chuck's voice is best on this album, but you can still sense that that he didn't know what he was doing with the guitars at some point.
Symbolic - New vocal style, not bad, this is most people's favorite Death album. Was mine for a while, but I kinda got tired of it because I listened to it too much. This album is a lot more progressive than their older stuff.
Sound of Perseverence - I hate the vocals, but the guitars and melodies are really great. Most melodic Death album, probably most suited for a Symphony X fan as well.
Hope that helps.