I've found that drinking bourbon directly from a flask takes away much of the enjoyment that the same bourbon would provide if sipped from a proper glass. Accordingly, I find myself leaning toward bourbons that have intense flavors. So far, my favorite is Elijah Craig 12 y/o, which had all but dropped off the screen for home sipping. It's not that I came to dislike it, it's just that so many others nudged their way ahead of it.
Anyway, what's your favorite, if it's different than your sipping favorite?