I have'nt read alot of sci-fi, but I would say the Yuuzhan Vong.
First, they slowly start to "Vongform" your planet seeding it with creatures which destroy local habitats and remake the planet with their crazy flora and fauna.
Next they come in their starships with overwhelming firepower.
Then, they invade the planet by ground, slicing your armies bits in their almost indestructible armor.
After that they destroy all native technology and enslave everybody.
Finally, by the time Vongforming is complete, the palnet is totally reshapen and the locals are mere chattel.
Yeah, that would really suck.