Serious Eats - seriouseats.com

Stick with simple: 1-2 parts tequila, 1 part lime juice, 1 part Cointreau, maybe a little bit of simple syrup, a dash of lemon juice. Put in a mixer with ice and shake.

My favorite tequila is Tres Generationes, from the Sauza group. I prefer blanco (a.k.a. silver) because it has a purer flavor than aged tequilas.

Despite the header on this link, there is a major discussion of good margaritas here:
http://www.tommyeats.com/tommyeats/2007/05/jalapenos_glen_.html

"...don't get me started on the margaritas these places make, which despite your not getting me started, I will right now. I'm sure that margaritas are one of the most popular cocktails served in the US, and there's nothing wrong with people sucking down cheap sweetened tequila and high fructose corn syrup mixers. It's just that I have a preference for non-gloppy-sweet margaritas, made with nothing but Tequila (100% Blue Agave), a non-crap orange liqueur, and fresh lime juice (and maybe a bit of simple syrup or OJ. Maybe). Let me stress again: FRESH LIME JUICE. And no, fresh lime juice can't be substituted with Rose's neon green high fructose corn syrup water stuff. That's not lime juice. So restaurants, please don't list "lime juice" on your drink menu if you actually mean "Rose's neon green high fructose corn syrup water stuff." That just throws me all the hell off. Or, God forbid, that sour mix stuff that comes out of the gun. Oh Jesus no.
And try to ask a bartender to make a margarita "without sweet and sour mix or Rose's"!??! You'll likely get a look of confusion and horror, a look of horror matched only by the look of horror on my face when I get a mouth full of synthetic sugar product when I'm actually expecting the magical combination of a Reposado, Cointreau, and lime juice.
Show me a place in New Jersey that makes a margarita like God intended it and I'll be on the road in minutes. Englewood's Mama Mexico? Nein. Bloomfield's Senoritas? Nicht. Norwood's Jose O'Reilly's? Nuh-uh. In fact the best margarita that I've had in New Jersey was at Ridgewood's multi-level meat market Blend, of all places. And this was due probably entirely to the excellent bartender Steve, with whom I had an educational discussion on and tasting of tequilas a while back. I'm not even sure if Steve still works there, but if you're there, and he does, have him make you a margarita with fresh lime juice. Accept no less."
And then...
"Bobby, the bartender, who is always always always there and has been since I can remember, will do his best to avoid the Rose's and the gun if asked, but it's a bit of a sales job to get him to keep his hands off of the stuff. As you might imagine, fresh lime juice is not prepared in bulk and behind the bar (contrast with Zarela in NYC), and having him go to the kitchen, grab a lime, bring it back, and squeeze it with his hands, seems a bit too much to ask, especially if you're, um, "thirsty." On this night I asked Bobby to go easy on the sweet stuff in the margarita and he did his best, even though he clearly did not agree with my approach. "You don't want Cointreau then," Bobby pointed out. I started to explain that I didn't mind *that* kind of sweet stuff, but stopped. The margarita was passable, but could have used about 7 more shakes. I like my margarita cold, well-shaken, and with ice chips. But that's for the next rant I suppose."

i usually use a mix of fresh squeezed lime and orange juice instead of that bottled crap. tequila (not rot gut), cointreau, grand marnier, the juices, over ice with salted rim is my favorite.