Don Juliio 1942. I bought a bottle in Puerto Vallarta at La Cas de Tequila when it wasn't even available in the U.S. The bottle is shaped like an agave leaf. Now it's available here, about $140 last time I checked. As smallg says, it's pricey, but worth having for special occasions.
My other favorite is 1921 Reserva Especial (anejo, too). For a cheap quickie, try Patron silver and Squirt for a pseudo-Margarita.
Try this margarita recipe (based on Adobo Grill margarita next to Second City in Chicago's Old Town on Wells Street) using a good reposado or even anejo tequila:
Add ice to a martini shaker. Play around with these approximate amounts of ingredients: 2-3 oz tequila
.75-1 oz orange liqueur (Grand Marnier....)
1.5-2 oz each freshly squeezed lime juice and sugar syrup.
Shake, serve up in cold martini glass (coat rim with salt if desired).
Important to use fresh lime juice; concentrate just doesn't do it. I make sugar syrup by adding 4 cups sugar to 2 cups near boiling water. After the sugar dissolves, keep heating for about 5-10 minutes and then cool. Store in refrigerator (I use Gatorade bottles). It keeps for months. It takes some work, but it's well worth it.