If you have Tali, be sure to upgrade her Powers in the setup screen - especially Sabotage and Combat Drone. There are three crosshairs around the star, but the Dreadnought is the one between the star and the first planet, Rannoch. The Geth Dreadnought is in the Tikkun system, the only system you can visit in the Perseus Veil. If you want Prothean lore however, Javik is a good pick, especially if you completed the Arrival DLC in Mass Effect 2. For unique dialogue, EDI will give the most by a long way.
Her Sabotage is an essential tactic against Geth enemies, and it's a good idea to double down with another tech-focused character like EDI or Garrus.
