I'm trying to remember, wasn't Leon a sorcerer before he became a spawn? Or am I getting that mixed up?
As for him being the best, maybe he had more skill than the others from his time alive. That wouldn't be surprising if he still had sorcerous powers. Or maybe the others had been tortured for so long that they just didn't care anymore about the special room privilege. Maybe Cazador simply lied to them and said Leon was "spawn of the month" to generate jealousy and competition for fun.
Yep he was a sorcerer.
but how did Leon get so good at being Spawn of the Year if he'd only been a Spawn for a few years at most?
He didn't have to actually be better than the others, Cazador doesn't do "fair". Leon had a mortal kid to somehow keep alive among starving vampires, giving him that privilege means he'll do anything without complaint to keep it, and live in constant fear of losing it anyway. When the spawn come to try and kidnap Astarion, if you explain the reality of the ritual to them, Leon's the one who first remembers that Cazador likes to give hope just to take it away.
While for the others it's the "youngest sibling" getting special treatment, undeserved.
