Incorrect.
While there are more powerful options for characters in FFVI, they don't come without a lot of leveling caveats, whereas Strago's Lore (Azure Lore if you will) comes in stronger than all other characters skill options relative to game progression with just his 3 starting spells.
His most powerful spell is tied to a World of Ruin sidequest that at most is locked behind just completing Owzer's House in WoR OR Veldt Cave (so one dungeon after Falcon acquisition, either way) if you let Shadow perish. This spell is basically a more MP efficient Ultima for all intents and purposes. It has slightly lower MP cost and spell power than Ultima, but easily reaches maximum damage before you can even acquire Ultima. This is maximum damage with his innate skills long before other characters can do maximum damage with their own innate skills.
Alternatively, his other most powerful spell is Step Mine (Traveler in later editions) which is a spell analogous to Quina's Frog Drop. Except it's tied to how many steps you've taken, and it's accessible within the World of Balance from early game enemies. You can get it right after getting him, and then spend like an hour or so in a town with Sprint Shoes on ramping it up until it is at max damage. Which is still faster than other means of acquiring maximum damage dealing attacks. And also used in minimum level runs since it doesn't rely on stats for its damage calc.
By comparison to other Blue Mages, Strago is a stand out in the category. Blue Mage in its inception appearance doesn't get to its game breaking spells until like, Karnak Castle escape, which is like half a dozen dungeons away from the Wind Crystal shard acquisition.
Gau is another great and comparable/arguably better Blue Mage than Strago, even having access to spells that cast with the Lore animation but Strago cannot learn. But he gets bogged down by how long it can take you to run into enemies on The Veldt, and that's if you didn't miss their encounters.
Enemy Skill materia is great, but it's not a character.
If it weren't for that one artist that drew the funny comic with Quistis, I'd forget she was a Blue Mage 90% of the time.
Quina comes in swinging hard, because you can get most all of her incredible spells right after recruiting her, particularly if you get her early. But I mean, this is mostly Limit Glove, which has that pesky needs to be at 1 HP stipulation. She shines harder in people's minds from being from a more recent game, but also from the fact that Strago is in a game where every character has to compete with Esper granted magic, so it's not uncommon for you to already have a few characters with magical prowess comparable or better than Strago's starting spells.
So going back to VI, with Esper Magic and making sure to get that Magic Power+ on level up from specific Espers (Maduin, Cait Sith, Zoneseek, and Tritoch) any character can get insane in the membrane magical damage, making Strago seem less special, but he can do that too and it also boosts the powers of his unique Lore spells. In addition he has gear access others don't and can get close to the same amount of Magic Power+ from gear that Terra/Celes/Relm get. Then there's always that funky layer that all Blue Magic has where he gets spells that do things that standard magic doesn't. Like he gets the Aero spell which is basically like a tier 3 on all enemies wind spell.
