The first Legendary Pokemon ever caught and used by a trainer in the anime was Brandon's Legendary Titans.
His Registeel was seen as more defensive than his Regirock and didn't get as many dominant showcases by comparison.
Gary Oak easily defeated Giovanni's first two mons and was over his head when he thought victory was assured.
However, it was also the only one of Brandon's Legendaries that was seen losing, as it lost to Pikachu in Pace
Nonetheless, Tobias infamously had Latios and Darkrai (a Legendary and a Mythical Pokemon).
Goh caught it in Johto, and although he wanted to release it, Suicune decided to continue being under his control.
The Battle Frontier arc featured a plethora of Legendary Pokemon, one of which was Noland's Articuno.
It was Tobias's ace, having easily defeated every Gym Leader and other trainers in the Sinnoh League on its own.
It was arguably his strongest mon, as it was capable of defeating Ash's Melmetal in the Manalo Conference.
Due to Melmetal accompanying Ash, it received far more screentime than most of the previous entries on this list.