It's a tie between the Naalu Collective and the Clan of Saar for me.
The collective with advanced fighters and their Race specific technology gives them the ability to control all movement throughout the galaxy. Send at least one advanced fighter to as many systems as possible. Nobody wants to pay two command counters and possibly lose ships to attack a fighter that may or may not choose to run away before the battle begins. Also by controlling movement you can make deals with players allowing them safe passage (As long as they have a diplomat with them). Tell a player they can move through an empty system you control to get to their real enemy by giving you some trade good or something of the sort that way they can keep their precious command counters.
The Clan of Saar is just sick. They can do it all.
The last game i played with them i ended up taking rex and using it as the base for all of my operations. Used diplomacy on rex preventing attacks from any threat while sending cruisers with stasis and type IV drives to take any open planets. Also try to steal pds systems and space docks if you dont already have 3 from your opponents rather than build them yourself this is a great strategy. attack a players planet with pds just for the sake of ripping the pds systems off and carrying them around with ur fleet. helps prevent attacks from destroyers. Then give them the planet back dont waste time trying to protect it. in fact dont protect any of your planets. Let them change hands you're the only one that gets paid when you take them back.