My favorite is "Comfortably Numb." Such great imagery and feel, and then punctuated by that guitar solo of rage and sorrow.
After that, it depends on the mood. If I'm feeling overwhelmed with work demands, "Welcome to the Machine" is good. "Time" and "Sheep" are always good for that too.
If I'm on the road, "On the Run," "One of These Days," and "Run Like Hell" keep me going.
If I get the time (both in terms of time to organize it and time left on the planet) to orchestrate my funeral like I want to, it'll feature "The Great Gig in the Sky," and not the one from "Dark Side" but the live version from (I think) "Delicate Sound of Thunder."
Yeah, too many great ones.
I'm not a huge concert-goer. Crowded traffic and crowds in general make me uncomfortable. But I've seen Pink Floyd three times, and they were simply mind-blowing. Stadium concerts, and it couldn't have been any other way. They could've named it "Walt Disney Goes to Hell and Blows Himself Up." Just a great, great show, and it got bigger and better each time I saw it.