Pappy's 23 years is best, and can convince even a Scotch snob that Bourbon is a serious drink. Right behind it is Willett 18. Pappy's 20 years is terrific, and almost an entirely different thing from the 23, more rye-like in being spicier.
Woodford Reserve, Basil Hayden, Booker's, Blanton's, Maker's 46 all offer distinctly different spins on Bourbon at daily prices, to equally good quality levels.
Not a Bourbon, but I have to mention Bernheim Original Wheat Whiskey for being an exceptional medium for ingesting alcohol, and it happens to be quite affordable.
Phil