Procyon Insurgency...the one where you need to take down two giant warships to beat the campaign.
I'd cheat that before, but now, with added experience, perhaps I can take 'em out...or not. Either way it's pretty hard taking out 2,100,000 hitpoints of enemy hull plating. The closest I've come to that is taking down entire Sathanas Juggernauts in Second Great War Part II, as well as knocking off every single turret in a wing of Juggernauts.
Still, it was pretty fun knocking down entire scores of cruisers, corvettes, and destroyers in a single mission, with 4 wings of pilots flying top-class fighters such as the Erinyes and Ares, bullying any Shivan fighter that comes close to me, while I repeatedly pound on enemy warships with Cyclops'es on my Sekhmet (arguably the best canon bomber I've flied).