nicohvc: Well maybe not invasion to the Earth. The Zergs (at the begining) had invaded Mar Sara, a Terran (humans) colony. Starcraft is my all time favourite videogame with aliens ^^ My favourite RTS and maybe my favourite all time video game... I don't know... easy at my top 5
I wouldn't qualify it as an "alien invasion", as, indeed, the implied subject is the "oh no martians invade earth" sci-fi cliché. On other planets its just, well, intergalactic wars with mere armies invading and occupying other territories. In spaaaace.
BUT
If we decide to go THAT way, then I would mention the "Creeper World" series as the best and most dreadfully oppressive version of "aliens invade alien planets". That's basically the blob conquering the universe as you try to fend it off with turrets and bombers. A sentient growing all-devouring liquid mass ? Beat
that, zergs !