Death Star(Star Wars)
The Death Star originated from the movie Star Wars back in 1977. The Death Star was built by the Empire and is capable of destroying planets, especially Princess Leia's home world, Alderaan. Interestingly, the Death Star also resembles a planet, or more technically, a moon, since it has a crater. This must have taken the Empire a long time to build this thing from scratch, and in space, no less.
Unicron is a Planet-Sized Transformer from the Transformers series. To my knowledge, he first appeared in the Transformers: The Movie, back in 1986. He also appeared in the Marvel Transformers comics, as shown above. Unicron's transformed form is a giant metallic "planet", but unlike the Death Star, Unicron is sentient and instead of destroying planets with a laser cannon, he devours planets as food. Another fictional character who also devours planets is the Marvel comics character known as Galactus(Shown below).
In fact, I think Unicron is pretty much the Transformers clone of Galactus.
I may do a comparison between Unicron and Galactus soon, but for now, let's just focus on the Death Star and Unicron.
According to a "fight" on Googlefight.com, there are more Google search results for the Death Star than there are for Unicron.
Also, according to an off-topic discussion poll in Gamespot.com, Death Star has beaten Unicron.