The entire Scrin airforce from C&C 3. Their main ships outrange your AA defences, meaning you have to use the mobile AA option (or Firehawks), and even then they have abilites (and those damned Stormriders) that'll gut them apart long before you reach them. Fortunately, I think some of this got toned down in later patches, but it doesn't stop the cheat way to win the Global Conquest mode of KW - have a force containing only Planetary Assualt Carriers and Deverstator Warships. Watch as you win a good number of auto resolve battles in a row (especially when upgraded!).
The Core Defender in Tiberian Sun. But he's meant to be overpowered, so your mileage may vary on that one.
A couple of things in C&C 4. But due to the NDA, I can't say any further than that.
Incidentally, any form of spam in current generation RTS games usually qualifies as a game breaker.