For me it would have to be the Battle of Garqi. The Remnant under Reborn Palpatine sent a fleet of 2 Sovereigns + a crap-load of various support ships (MTCs, Ton-Falks, ISD Is, ISD IIs, Strike Cruisers, Lancers, VSDs, lots of Vindicators) to Garqi after a number of failed assaults on some of my other planets. When the battle started, I had 1 Praetor II, 2 ISD Is, 2 Venators, 2 Acclamators, 1 Munificent, 1 Enforcer 2 Imperial Escort Carriers, 1 IPV and 1 Hypervelocity gun on the surface (note, I play with the Balance and Flavor submod).
The first thing the Remnant did was use their Sovereign (lets call it "Decimator") to disintegrate my Praetor, leaving my fleet without a command ship. They then rushed forward with their support ships, attempting to overwhelm my defense, but my line held. After a long while of frigates smashing upon my defense, the Remnant decided to stop calling in reinforcements which I found to be odd, but I continued destroying ships.
About halfway through the battle, they had enough room to call in the second Sovereign (lets call it "Punisher") which annihilated one of my Golans with its superlaser. I hadn't even knocked out the shields on the Decimator yet so I was very concerned. I decided it was time to stop letting them come to me and to take the fight to them. My cruisers moved on the Punisher while any fighters with torpedoes or missiles attacked the Decimator who was wounded by multiple HVG shots, but still kicking major aft. Thankfully due to EAW pathfinding, the Punisher turned at a bad angle, leaving the aft of the ship exposed for my cruisers to advance and hammer it. Once my cruisers were in range, the Punisher's shields started dropping fast. After a good couple of minutes of barrages, the Punisher decided it had enough and turned to annihilate one of my Venators, both of my Acclamators, and my Munificent. Meanwhile the Decimator had taken significant damage due to being consantly pounded by V-19s and Skiprays so it tried to fall back. Unfortunately for the Remnant it didn't work out so well and the Decimator was lost to old fighters and bombers. Though the Decimator had been destroyed, the Punisher was still fighting, albeit a losing battle at this point. eventually the Punisher fell to my fleet, nearly taking one of my ISDs with it. After that it was just a mop up operation of any surviving Vindicators, VSDs, and Strike Cruisers.
My total losses were the Praetor, 1 Venator, 2 Acclamators, the Munificent, the Enforcer, the IECs, and the IPV while the Remnant lost everything. It was a glorious day for the Pentastar Alignment.