Katatonia pretty much stopped playing doom after Tonight's Decision (which isn't really a doom album either, but it has some doom elements left), but pretty much their entire discography is amazing. If you're into full-on doom, I guess you should try the first two albums (Dance of December Souls and Brave Murder Day), but if you just want to try the band, try either Viva Emptiness (darker, catchier) or Night is The New Day (lighter, even slightly prog), and then The Great Cold Distance which needs a bit more time but is one of my favourite albums ever created.
Another recommendation - a band that's seriously underrated here on DTF and all my attempts of converting people have failed miserably, and that is Paradise Lost. The first five albums are all pretty much doom, although with each album there's a bit less doom and more gothic. Their later '90s albums are very experimental and a bit electronic (One Second, Host, Believe in Nothing, Symbol of Life), self-titled is more straightforward, and with the previous, In Requiem, they returned to the doom/gothic style though a bit different. However...
Faith Divides Us - Death Unites Us was released in 2009 and it's absolutely amazing. It has plenty of both doom and gothic in it, and I recommend it all the way. It was my #2 album of 2009 (behind only The Incident) and it's pretty much flawless.
That's all I can say. Unfortunately I've wanted to get into more traditional doom for a long time but been postponing it for a while already.