I've only seen DT twice, but unfortunately they haven't played any of my top 4 tracks - LtL, Metropolis, Octavarium, ItNoG, so any of them would be amazing. I'm desperate to see Learning to Live or Metropolis especially, as they've been my favourite songs for ages now. I seem to have just missed both of them as they'd been played on the legs either side of the shows I went to. I feel bad for after seeing cram's post above though - 8 times seeing them live without playing your favourite song must suck! Especially when it's a fan favourite and not played irregularly. I have a funny feeling they might play Take the Time instead on the upcoming tour though - while it's an awesome song, it's one that I have seen before.
Basically anything from my top 20 or so tracks would be great to see live, but aside from the 4 I mentioned earlier, here are a few I'd love to see:
A Change of Seasons, Trial of Tears, The Glass Prison, Sacrificed Sons, The Ministry of Lost Souls, Lines in the Sand. There are a load more of course, but these are ones that I haven't seen before and haven't been played for a while AFAIK