what's the best bourbon to put in eggnog?

I rarely put the good stuff in my cooking (with the occasional exception of wine), especially if the other flavors of the dish are pretty strong (like in eggnog). I just don't find that subtlety comes through, so why spend the $? Would rather savor the good stuff on its own than cover it up.

Have a favorite eggnog recipe to go with that?

I agree with Zelda. No need to go expensive. I prefer a brandy or dark rum in eggnog, but never anything expensive.

For what it's worth, I'm a formally educated wine and spirits professional. I defy anyone to taste the difference between a cheap spirit and expensive spirit when it's mixed (and especially so after a glass or two...). Feel free to disagree, pros or amateurs. I would buy something cheap-to-mid-range for eggnog. The good stuff is a waste in an eggnog.