I'd have to say Knockin' on Heaven's Door, Day Job, IFTL, or maybe even Bid You Goodnight. Now, I love BYG, but not as an only encore. I don't mind when it's paired w/ another song, but not solo. That's just not fair. Also - I love the song, but I hate One More Saturday Night as an encore, because if the 2nd set comes to a close and they haven't played it yet, you know it's coming, and that just plain sucks because it spoils the whole anticipation, which is one of the best parts of the encore. I know every one of us has been at a show, all giddy as hell, and turned to their friend and said something to the effect of: "oh boy oh boy oh boy, I wonder what they're gonna play!?!?!?!?!"
Actually, I just thought of another song I like, but not as an encore: Baby Blue.
Not to keep dragging the proverbial step-child into the conversation, but one thing I really liked about Phish is that they didn't have (for the most part) a "set" setlist like the Dead did. They'd play just about any song anywhere, including as an encore, where the Dead had a set "set" od songs that they'd do (esp. during the later years - Brokedown, Quinn, IFTL, Liberty, Box, Black Muddy, Baby Blue, Sat. Night, US Blues, The Weight, Knockin', etc.). Now, once in a while you'd get lucky and get a Touch, Revolution, or something tasty, but especially the later years, I bet you could've made a lot of money betting some not-so-heady-heads on which song they were gonna encore with. At least with Phish, they'd mix it up a lot more. Just sayin'.
You know what I mean, though? You just knew there was no way you were getting a Jack Straw, Bucket, Crazy Fingers, Playin', or Dire Wolf encore. Not a chance.
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"We must embrace pain and burn it as fuel for our journey" - Japanese poet Kenji Miyazawa