I've experimented for a couple months now with two different flasks. One is of conventional shape, with capacity of either six or eight ounces -- I forget which. The other is the nifty little Jameson-imprinted one, which looks like a jumbo pocket watch and holds about three ounces.
I have found that drinking directly from a flask changes the experience considerably, as compared to drinking from a glass. Even a shotglass allows some appreciation of the nose. The flask all but eliminates it.
Accordingly, I have been reluctant to fill a flask with any of my more complex and enjoyable bottlings. In fact, I briefly went the other direction, as I tried Kentucky Pride in the hope that drinking it from a flask might mask some of its shortcomings. It didn't work.
I tried Jefferson's Reserve and found that the pencil-shavings flavor was thankfully diminished, but I missed out on the pleasant nose that a proper glass reveals.
Of the six or eight bottlings I've tried, my favorite flask-bourbon so far is EC12. Crudely put, it has the balls to provide a full-fledged taste, even when poured directly from the container onto the tongue.
What preferences do you have?