Im using XO
I use Brandy w/ egg nog, but I use VS or VSOP for that kind of stuff.
My rule is XO or older for sipping for MYSELF!
VSOP for mixed drinks for myself or for sipping with others.
VS for cooking or for mixed/drinks sipping for people who wouldn't appreciate the differences between VSOP/XO, etc.
That's alls I'm saying.
By the by, I also use brandy (not good cognac though) for egg nog.