John Myung tackling James LaBrie in the middle of a show on a bet.
Mike Portnoy playing drums with one hand while playing catch (using his own drumsticks) with one of the sound guys offstage in the MIDDLE of "Erotomania".
During the Progressive Nation tour on 2009, Mike Portnoy did drum solos with the drummers of each of the bands touring with them - Landryx of UneXpecT, Steve Frothingham of Bigelf and Martin Axenrot of Opeth. After individual solos with each drummer, ALL FOUR of them played at once on Mike Portnoy's gargantuan drumkit. It has to be seen to be believed.
"This drumset is too big for me alone."
Dream Theater had attracted criticism that they were trying to sound too much like Opeth with Portnoy's shouted vocal parts on more recent albums. So, in that same show, Mikael Åkerfeldt came on stage to do the shouted vocals in "A Nightmare to Remember" with his characteristic growling. Check the awesomeness.
Score, where an entire orchestra performs the overture to "Six Degrees of Inner Turbulence" made it all very much worthwhile. Not to mention any and every thing that followed said moment.
The entire "Live Scenes from New York" concert: they played the entirety of Scenes From A Memory, another full concerts worth of music after that (including all three parts of "A Mind Beside Itself"), and they for the encore, they played the 23 minute long "A Change Of Seasons". The icing on the cake is at the very end when James Labrie apologizes for having such a short set. Even more impressive, Mike Portnoy was suffering from food poisoning the entire time, and was hospitalized after the concert. And it still sounded awesome.
Any and all of James LaBrie's highest notes, but most notably 'trapped inside this octavarium!' (raised to an epic descending G5 on Score... with orchestral backing) and of course the legendary F#5 from Learning To Live.