In the anime
As is often the case there were two lesser features of Garchomp with core characters and then one final one that put it on its legendary pedestal. First, Professor Sycamore longest tenured partner was a female Garchomp that had the ability to Mega Evolve. She made several appearance alongside the professor throughout the XY era and was quite powerful in her own right.
Another Garchomp that showed up on multiple occasions partnered with Ash in Alola as his Ride Pokemon during the adventures of the Ultra Guardians. Not much is known about this Garchomp but it was still fun to see it partner with the series hero.
No disrespect to the other characters, but when you think of Garchomp in the Pokemon story you think of Cynthia, the Champion of the Sinnoh region. Used as her main battling partner, Garchomp was shown to be quite resilient and powerful. Most attacks, even from powerful Pokemon, had little effect on the champion’s ace partner and was a regular ally for Ash and Dawn to rely on in battles against Team Galactic.
When Cynthia made the trip to Unova of course we got to see some Garchomp action, this time taking on Iris and Axew. She also battled against Unova Elite Four member Caitlin and her Gothitelle in an exhibition match. In one of the few times Garchomp was pushed to the limit the battle went back and forth before being forced to a time-limit draw.