While not the only way, one of the easier ways to dominate the entire galaxy is to first go through the Polaris storyline. The best ship to use as escorts tend to be the carrier-class vessels. In general, the Polaris technology is much more powerful than that which youcould aquire from other storylines, and will therefore be a better and easier start toward planetary domination. Edit
With those things in mind, the best escort to use in your quest to conquer the galaxy is a Raven because it is the most powerful Polaris carrier-class vessel. Personally, I do not like the way the inertialess drives handle, so I prefer the Kestrel as the player ship with Ravens as escorts. But be aware, the Raven is much more customizable.
If you do choose the Polaris storyline, and wish to use Ravens as escorts: The best, and by far easiest way to secure raven escorts is to: Edit
1.) Purchase a Raven of your own (a paultry 8 million credits)
2.) Capture a Sprite freigter.
3.) Upon capturing the freighter, it will ask if you want to keep the Sprite as your own ship or as an escort.
4.) Select "Use as my ship" and you will retain your old Raven as an escort. After you capture the Sprite, and have your new Raven escort,
5.) Return to port and purchase another Raven.
6.) Once you have your second Raven, sell all the valuables in the outfitter, especially the Mantas. After which you will have slightly more money than you started with. (I personally like to use the Biorelay Laser Turret to capture the Sprite)
7.) Repeat until you have all six escorts as Ravens.
You will need to have a good reputation with the Polaris government in order for this to work. By the time you are able to purchase Ravens, your reputation should be untouchable, unless you've done something to anger the government already (e.g. Domination)