For me, it always comes back to the Platinum Music Awards ... there are other moments that are important, for sure, but that's the one that always is the critical one to me.
Canon is a huge mess, in a lot of ways, and it barely makes sense sometimes, because it is very much a sitcom and not so much a plotted out show? So there is very little continuity or character development or internal consistency from episode to episode in terms of character traits.
There are definitely a few moments that stand out as important relationship moments for these two - the end of Stage Fighting where Jade comes to find Tori in detention and the entirety of Play Date are both big ones, for example. But for me, it always comes back to the end of Tori Goes Platinum for a couple of reasons.
I've talked before in fanfic about how The Worst Couple is kind of what I see as the critical moment for Jade - before that, she's with Beck, and I see her personality as not tolerating cheating, either from her partner or herself. So up until that moment, near the start of season 3, she wouldn't really be looking at Tori. And the PMAs feels like a significant emotional stakes moment for both Tori and Jade, where their relationship moves forward, where they leave themselves vulnerable and make a commitment to each other. The final scene between them backstage really feels raw in a way that a lot of the other episodes in the series don't. I think that's the turning point where things go from "friends" to "there may be something there that wasn't there before", to quote a certain movie.