Here's the go-to recipe for guacamole by one of Sacramento's most popular proprietors of tacos, Lisandro "Chando" Madrigal. He recommends using Roma tomatoes, which are less juicy than others, for this recipe.
1 garlic clove, chopped fine
1 tomato, diced
1/4 cup onion, diced
1 jalapeño pepper, diced (use half a jalapeño for a less spicy recipe)
2 tablespoons cilantro, chopped
Salt to taste
Cut avocados in half and remove pits. Scoop out avocado pulp and add to molcajete, or other mixing bowl, with garlic. Mash to desired thickness. Add remaining ingredients and combine. Add salt to taste, approximately 1/2 teaspoon.