Yukon Jack
I have two. In one I carry Jameson, and in the other, I carry either Damiana or Celtic Crossing.
Brandy for me. That way if I ever run into somebody with cherries and someone else with vermouth, we can have manhattans aplenty. Or, you know, I can just drink the brandy.
Maker’s Mark usually. Sometimes Buffalo Trace.
I own a bunch of flasks. A couple are Russian (1, 2) A couple are just generic 8 oz. with keeper tops. One is a freebie from Gallo. All of them get used.
Early Times or Jim Beam, which both mix well with cola, which is always for sale at concerts, games or movies…
To save $$$ while bar hopping, Patron or even Jose C. is a good bet, since its always easy to get a lemon wedge or two from a bartender on your way out the door.
I’ve always :dubious: at flasks. I mean, that just ain’t enough alcohol to make a dent in reality. Give me one of these filled with JD, baby!
Joe
Drambuie is the current resident of mine.
ahahaha
To answer the question, I quit drinking in 2007, but before then, 100 proof vodka.
My flask o’ brandy is just for enjoying the occasional sip or two, not to get me plastered. I’m pretty sure a litre of brandy would put me in the hospital, if not kill me outright.
Sailor Jerry’s Spiced Rum (and a casual raise of the flask to the OP)
“Grape soda” 
I haven’t been down to my vast, secret laboratory lately. Must I use a flask, or will a beaker do?
I don’t generally keep my flask full, but do use it for special occasions. Bushmills for light hearted nips, or Smirnoff Blue for serious business.
Goldschlager or gin.
Wow; quite a variety. Wouldn’t it be neat if we were all on a bus, and it broke down somewhere, and to pass the time we all spontaneously pulled out flasks?
I think I started thinking about this because a flask was featured on Battlestar Galactica, which inspired me to get out my own flask, which now needs refilling.
Fill it with vermouth, we’ll make an evening of it. 
Hrmm…just how common is this flask-carrying thing, I wonder?
I would have thought that flasks were just for alcoholics–the only two people I’ve ever met who had them were serious alkies–but I’m hearing some self-reported flask use from people here who I consider to be responsible drinkers, and the rest are just people I don’t know anything about, drinking-wise,* so the overwhelming effect of this thread on me is to make me question my assumptions about the damn things.
And also, to make me kinda want to go out and get one, even though I hardly ever drink these days! Bombay Sapphire for those long study breaks? That would make nights at the library slaving over a hot sentence tree go a little faster, wouldn’t it? 
- Plus one recovering alcoholic, but I don’t know if that’s my Big Reveal or if it’s just the Law of Averages at work.
You forgot “puke” between “friends” and “when”.
Eh, I only rarely use mine, and I think a lot of folks like that. The only times I’ve used mine was taking it camping or an outdoor event or maybe a BYOB situation. Most of your hip pocket flasks only hold enough for like three drinks, anyway.
So yeah, I could see how an alkie would use one to maintain, as well, but they probably need more than that to keep going.
Phone number for AA…?