Science says:
Sipping them neat, here is my ranking.
Casa Noble anejo >> Avion anejo > Don Julio anejo >> Herradura anejo > Espolon
The Casa Noble is just so good. The vanilla and butterscotch flavors and sweet smell make this a really wonderful sipping tequila. Also, it is really smooth. There is no contest in my mind that this beats the others.
The Don Julio and Avion are both really smooth but the Don Julio is a little too subdued. The Avion has a brightness to the flavor that is really nice. Maybe somewhere between a good anejo and reposado? I am not so familiar with reposado though. Both very nice sipping tequilas.
The Herradura and Espolon both have a bit of a funny smell to them. They are both pretty peppery. The Espolon is much harsher than the Herradura. I bet it would make a fine margarita though and would probably be a great choice at <$20! (That is 20 dollars not 20 factorial dollars which would be absurd.) I wouldn't really enjoy sipping either of these though (especially the Espolon).
Oh yeah, after tasting these, I really want to try the Casa Noble reposado now. That will probably be my next tequila purchase.