There have been similar threads on the forum, but I guess my duty is to answer you once again.
Dark Tranquillity in short (lack of excess time on my behalf evident):
Skydancer - Their first full-length album, recorded back in '93 in an inferior studio compared to Fredman (where the later albums are recorded) - thus the somewhat bad sound and the band isn't fully mature yet, the playing not that "tight". The songs, however, are refreshingly original and well-constructed compared to anything ever done by any band - emotional and melodic, with little repetition and poem-like lyrics. Listen to songs like "Crimson Winds" and "A Bolt of Blazing Gold" and you get the idea of this unique album.
Of Chaos and Eternal Night - The Mini-CD made between albums and after some changes in the line-up. Very exploding, guitar-driven stuff. Now available on the re-released Skydancer.
The Gallery - The legendary album that set the standard for melodic death metal for several years. The production is excellent and the band very strong and confident. The two most popular songs of the album are the furious opening-track "Punish My Heaven" and the haunting "Lethe". A classic album, all in all.
Enter Suicidal Angels - Three songs taken from the studio-sessions of the following album, plus one horrible HC-techno-remix. Recommendable only to true fans of the band.
The Mind's I - The most brutal DT-album, with profound aggression all through the album. This might be the "extreme" stuff you are looking for. Majority of the songs are rather short and fast - "Dreamlore Degenerate" is a good example. Many would recommend "Insanity's Crescendo" to you, but it really isn't representative of the album's general direction. Neither is "Hedon", but that song is a classic everyone needs to hear.
Projector - A step to a new direction - clean male vocals once again (some where used on Skydancer) and less aggression overall. Great "moody" album, nevertheless, and there's still enough death metal elements to ridicule any Depeche Mode -comparers. Search for songs like "To a Bitter Halt" and "Nether Novas".
Haven - The latest album, and once again a new path - this time with a lot of electronics in the sound. The album full of instant hits, this album has grasped the band a renewed audience. Try songs like "The Wonders at Your Feet" and "Rundown".
Awaiting the next album...
-Villain
