I actually forgot writing a more extended version of why they're one of my favourites, which is pretty important, I believe...
The most important reason is that this was the band that showed me a whole new world of music. I listened almost only to heavy/thrash/power metal before (with some exceptions, U2 being the greatest and most important one
), and I approached Octavarium as a metal album, even if a bit different. Over time I realised how different - and interesting - this genre is, and my favourite, being Panic Attack for a long time, changed eventually to the title song. I listened to almost only DT for like half a year, I think (getting SFAM 3 months later had a big influence on that), and then started getting interested in other progressive bands. Also, DT made me look at music in a different way - not just the song as a whole, but I actually started to notice how each instrument works, I started to see the difference between guitar and bass
and just started to look deeper into music. Now, obviously, I listen to many other bands, but DT still has a special place in my heart, and probably always will have. There are four bands I absolutely love, each for different reasons (one is common for all four, though: a lot of depth and atmosphere in music), and DT is definitely one of them.
Over time I also learned that there is plenty of interesting music outside of metal, but that, young Padawan, is a different story...