On the concerts I've been to, the openers were...
June 2007: Overture 1928/Strange Deja Vu
June 2009: In The Presence of Enemies Pt. 1
September 2009 (PN09): A Nightmare to Remember
I think Overture 1928 was the most awesome of them all, I still remember that - the intro from tape, and then MP appears... Simply amazing. ITPOE1 was very surprising and was quite nice as well, although maybe the least powerful of all three for me. ANTR - the definition of "epic".
If I had to choose something else, the ideas I get are Pull Me Under, The Glass Prison, The Root of All Evil and Constant Motion. I think TGP would be my favourite (although this may also be affected by the fact that the only song from Six Degrees I've heard live this far was Solitary Shell
).