Vasari is best for hit-and-run tactics and "fighting dirty". Advent is supposedly best for slug-it-out battles. TEC is best for wars of attrition.
I personally find the Vasari the best for slugging it out against huge fleets because of the carrier cap's Repair Cloud which, although seemingly mediocre, is actually quite powerful. You dont need to split your fleet into offense and defense fleets since phase gates mean you can get to anywhere fast from anywhere, so oyu can commit more ships to a battle. And while Advent might have more firepower per ship (Crusaders are amazing), Vasari can use Subverters to shut huge groups down. If you micro them well you can keep them out of the fight indefinitely.
Vasari are like modern generals. They focus on minimising casualties first and dealing damage second.
Advent are like boxers. Very strong and focus on dealing damage first and healing second.
TEC are like those 12 year olds in Counter-Strike. Not good at staying alive or killing enemies, but no matter how many you kill they just keep coming.