These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Game Breaker: The Bulwark-class battlecruiser and TIE Defender are the two main examples. Early on, the Alliance Escort Carrier is a good example too.
The Assault Frigate is one of the Rebellion's first research results and it significantly tips the balance of fleet power in the Rebellion's favour (and the Empire doesn't catch up until it gets Strike Cruisers). An assault frigate is cheap, easily manufactured, and more than a match for the Imperial Star Destroyers that make up the mainstay of the Empire's early fleet. More or less as soon as the Rebellion starts putting out the Assault Frigate, the space game is reset as most of the Empire's existing ships will be rendered worthless.
Han Solo. He and any characters (not special forces) travelling with him move at double speed as they catch a ride on the Millenium Falcon. This mobility gives rebel characters a huge edge in the early game, as they can move to places of interest in the galaxy far faster than the Empire can react. A popular early game tactic is to send Han and a group of combatants to incite an uprising on whatever world(s) the Empire controls that are loyal to the Rebellion. Unless the planet is in the Sesswenna Sector (with the Emperor nearby and enough Imperial power to curtail the attempt), or the Empire is lucky enough to have a diplomat in the area that can move quickly to improve the system's allegiance, the planet can very quickly fall into uprising and take most of the rest of the sector with it.
Capturing Vader or Luke can be this because they have insanely high Combat statistics that only get higher with time.
Also, if you're a Rebel, any battle in the shadow of a Death Star, complete with Death Star shields, and a pretty sizable fleet — with a lot of TIE Defenders, of course — and Interdictors. Yeah, you can say good-bye to that fleet. If it's all around Coruscant, you need to do it to win the game.