My guacamole is a little different than most; but it's the way a lot of people make it around here:
3 medium-sized avocados
3-5 cloves of garlic, chopped (depending how much you like garlic)
A splash of lemon juice, to taste
Salt, to taste
Two or three good shakes of tabasco
Cut up the avocados and mash the pieces, stir in the rest. It'll get smooth eventually, but I usually leave it slightly chunky. This is a really simple recipe, but I've never had any other guacamole that I liked better.
Hass avocados (the ones that go black when ripe) are favored for guacamole because they have the highest oil content and make the creamiest guacamole. But "creamiest" isn't necessarily "best tasting."
Some green-skin avocados make very fine guac because of their superior taste: particularly the Gwen, the Pinkerton, and the Reed. These are often very firm-fleshed avocados, but you'll know they're ripe if they have a faint speckling of black across the green skin. Even if they seem too hard, they're ripe, and the flesh will be fine once you slice them open. Because they're firm, they also bruise less than Haas avocados.
Stay away from Fuertes, Bacons, and Zutanos for guac; these are all big, bright green avocados with stringy, pale flesh that doesn't taste like much of anything. Even when they're ripe, they don't taste ripe!
Just for the hell of it, here's a pretty hard-core avocado site out of the University of California at Riverside, which has an avocado research facility:
http://ucavo.ucr.edu/
Click on the Varieties button to read a pretty good article on where the Hass avocado came from, and another article on some of the best greenskin avocados and why they might supplant the Hass sometime. I buy Gwen avocados (green skinned) from a grower at the local farmer's market. He grows Hass, too, but he constantly encourages people to try the Gwens because he says they're better. And he's right.