LTL.
Metropolis has been my favourite song by anyone for quite a while and my favourite DT song for even longer. Learning to Live has always been close behind but recently I've realised that I actually like LTL better. Just checked my Last.fm stats for the last 12 months - listened to Metropolis 46 times, but LTL 70 times.
Both are incredible though and I don't think DT will ever top either of them.