I'm going to say Rush.
I've watched a lot of their music videos on youtube, and bought 2112 on iTunes. Yes, I can appreciate their abilities, and I can see the influence they have had on DT, but I'm still looking for that "gateway" song that makes everything click for me, and gets me hooked.
At first I tried with 2112 and Hemispheres (Those were the ones a friend recommended me to listen first to) but they didn't quite get to me. I found myself liking their songs via Clockwork Angels and Vapor Trails. I don't know if that's a proper or 'right' way of getting into them though
As for the bands, well...
Nightwish. I don't know, man. Their songs seem too over-bombastic to me I guess. Maybe I haven't listened to them enough.
Avenged Sevenfold. I don't really like M. Shadows voice, sorry.
Destruction. They pretty much made me realize how same-y Thrash Metal could become. It's not that I don't like German Thrash Metal, since Sodom is for me one of the best Thrash Metal bands in history; and at least I can get into a few Kreator songs, but... I can't really enjoy their songs.