Everything's a matter of personal taste, no?
But if you respect the pros, the highest combined rating for a tequila around $25 is Milagro silver. It pulled down a gold medal from San Francisco in 2009 (a double gold in 2008) and 90pts from the Beverage Testing Institute.
The top 10, according to the pros we follow, are:
El Tesoro Platinum ($40)
Trago Silver ($40)
Gran Centenario Plata ($43)
Fina Estampa ($30)
Espolon Silver ($35)
Don Eduardo Silver ($40)
AsomBroso Silver ($50)--this one's worth the extra money just for the provocative shape of the bottle.
Jose Cuervo Reserva de la Familia Platino ($60)
Milagro Silver ($25)
Corazon Blanco ($45)
I personally either go with Milagro or, since it's a little more readily available, the perfectly acceptable 1800 silver.
Regarding that list: Someone gave me a bottle of Reserva de la Familia Platino, haven't tried it yet. Should be fun. I've also had Milagro Silver through at least a couple bottles worth. It's good for the price. I certainly liked it much more than the 1800 Silver that I started out with.
But personally I've discovered I like reposado more, even in margaritas. Lunazul reposado is inexpensive and works just great for a margarita, I found. Can be had for $20 some places, maybe less. Here in PA we tend to be somewhat restricted in choice because of the state-run liquor stores, but they do now carry Lunazul reposado, and I think it's the least expensive 100% agave in the store (cheaper than 1800, Sauza Hornitos, etc). Sounds questionable, but for margaritas especially, it's just fine IMO. Of course, if you're going for the sharp agave taste of the silver, this isn't going to cut it. They do make a silver as I recall, but it doesn't seem to be as widely distributed as the reposado.
I'm with you there: I vastly prefer the reposados. Like them better than the anejos, actually.
But that being said, I should've metnioned above: for margaritas for a crowd--that is, if you're going to dump the tequila with Mr Ts syrup mix and toss it in a blender--just buy the biggest plastic bottle you can find off the lowest shelf in the grocery store.
I'm the snootiest guy I know when it comes to liquor. But even I don't bother to spend much money for blended, crowd drinks. So I'm with you on the Lunazul. In fact, that might be too much money in my opinion.
But if you're going to mix up some classic margaritas on the rocks, I think the 100% agave of some kind or another is worth the investment.