Almost everyone I know is using the MK IV Engines from the "City at the edge of Never" quest line... even after they reach admiral, as the +5 to AUX, Weps and Shields outshines any other engine currently available in the game. Extra impulse speed is neither here nor there compared to 15 extra power for free.
QFT. No way I'm giving up the +5 Weapon/Shield/Aux to fly a little faster.
Not the answer you'd expect but, the Impulse MK VI is great for crafting. They are the cheapest to craft in terms of anomalies needed, and are worth 41k afterward same as the other kinds but take far less samples to make. So if you are trying to unlock tier 3, this is the way to go.