The Most Indispensable, Yet Disposable Pokémon Ever

Every now and then in a Pokémon game you come across a new addition that's just the thing you needed to overcome a hurdle, but then that saving grace becomes utterly useless almost immediately after its one shining moment! So let's look at the Pokémon everybody loves to catch, and appreciate them for their crucial status, but then sideline just as quickly, leaving them in the PC box forever...