You might as well ask what fruit is best to make a pie with! It realy depends on the beer, how much you want to spend on bourbon, if you're going to buy a 750 and drink the rest or want something in a 50ml bottle just for this, etc, etc.
Since two folks have already mentioned using French oak, I'll go ahead and point out that bourbon is aged in American oak barrels. I'd suggest going for any char level depending on how much of that flavor you want, but also boiling the oak for a minute or two before hand to leach out some of the oak flavor. Most bourbon barrels have held liquor anywhere from 4 to 15+ years before they get used for beer, so they won't impart nearly as much wood flavor as new oak cubes potentially can.