ANSWER: Roma (plum) tomatoes are the best choice because their thick flesh and low moisture prevent soggy, watery guacamole.
Roma tomatoes have a high ratio of meat to seeds, which holds a clean, firm dice that stays crisp against rich avocado.
Always slice them open and scrape out the watery seed pulp before chopping so excess juice does not dilute your seasoning.
If you cannot find firm Romas, sweet grape tomatoes halved and seeded are the next best alternative for great texture and concentrated flavor.