This Roger Corman-produced film I have barely heard of but according to Wikipedia a Stellar Converter used in this film can heat a planet's core "causing a delayed explosion where [the planet] incinerates from the inside/out".
Aliens, such as John Travolta, from the planet Psychlo have managed to conquer Earth, but in the end of the film a human resistance group is able to teleport a nuclear bomb to Psychlo, which – thanks to its interesting atmosphere – explodes.
I haven't followed the Dragon Ball franchise but nevertheless am aware of that blowing things up is nothing special in this universe. The 5th DBZ is spectacular enough to be started with an explosion of a main charcater's home planet.
After couple of sequels things really started to get out of control. The 8th DBZ movie begins with an annihilation of an entire galaxy. I guess that deserves some sort of honorary nomination. Thanks to Pumpkinate for the tip :)
A CGI bug (later Lara Flynn Boyle) wants to tell the audience who's the villian of the movie, so when travelling across the galaxy she destroys several planets in different ways – during the mere opening credits sequence!
I have no idea what happens in this film, but according to TvTropes in the end "the Earth explodes because the characters cut the wrong wire of the remote controlling all of the world's computers, that were turned into bombs".
Kurt Russell is one cool guy. A wimpy Colonel needing a spectacular end kaboom can't stand this fact and orders destruction of the waste disposal planet Russell is on. (Russell can't naturally be killed this easily.)
In the awesomeness that is Star Trek II: The Wrath of Khan one of the subplots is a mysterious Genesis device. When ignited, it can create life on a dead planet. Unfortunately, the whole thing is very unstable, and the new planet collapses in few weeks. Because William Shatner is awesome, he fights a Klingon commander (played by Christopher Lloyd) on the planet when it's effing collapsing.
Malcom McDowell is a man who will stop at nothing to reach his goal - a one-dimensional cliché in Star Trek films. In the 7th film of the franchise obession of the villian is so powerful that he sacrifices entire star systems to gain what he wants. He blows up two suns, with a single missile each. The missiles contain something called trilithium, which causes the nuclear reactions within the star to cease. This creates a shockwave capable of destroying planets. Though two suns are bombed, we see only one planet being ripped apaart by the shockwave.
In a JJ Abrams-helmed Trek film we're introduced an alternate timeline of the classic franchise. Vulcans couldn't stop the destruction of the planet Romulus and the surviving timetravelling Romulans are pissed off. They drill a hole into planet Vulcan's core, launch there something called red matter which creates a black hole. Vulcan collapses inside itself.
The evil Empire officers threaten to blow up an innocent planet called Alderaan by Death Star, unless Princess Leia provides them with some information. Leia gives up but Alderaan is blown up anyway. Why? Because the Empire is evil, of course.
In this feature film long toy commercial an old line of toys is wiped off to introduce a new one. In this process an important assitance is a ridiculously big Unicron, voiced by Orson Welles in the last movie in which he contributed. In the very opening sequence his character wolfs down an entire robot planet, just to feed himself, I suppose.
Listing films where a planet is destroyed. Not only is everything on the planet killed but the planet itself is blown up, too, or at least destroyed in a similar way.
Films are in alphabetical order, except if there's more than one entry from a film series - in that case, they are in the order of release. Feel free to tell about movies that should be included. Sorry for my monotonus English.