KH3 would be his strongest. KH2 Sora had more spells at his disposal, but KH3 Sora's feats go way higher. Plus with Formchanges, Grand Magic, Shotlocks, Attraction Flow and Flowmotion, he's way more versatile, especially if we're talking ReMind Sora. (Think about it, he did the end-game dungeon TWICE, and with a weakened heart.)
KH2 Sora had Drive Forms, but they required party members and tied to those clothes specifically, so if Sora has to go it solo, he's staying in base (this is assuming the gameplay mechanic also has actual active lore in it as well). And that cutting a building thing at the end of KH2 isn't usually taken seriously either, because Sora's never really out here cutting buildings in half and whatnot (he hasn't needed to), and that realm wasn't following the usual rules of physics.
KH2 Sora ain't a pushover, but thankfully it's not his peak.
You know, I wonder why he just hasn't straight up said "Thank You, Namine" yet. It being this drawn out makes no sense. The only reason why it's still a thing is because maybe that's gonna come into play in the future for some reason.
In fact, if we're dealing with the "Memory of Sora"... Who's the one person that's quite literally an expert on memories and Sora? Namine. My theory is that Namine will play a big part in Melody of Memory because of this, and she'll help Kairi trace the connections in order to find him. And the reason why KH1 Sora is the one we see often (besides marketing) is because Kairi was in Sora's heart in KH1, so this is probably the easiest method for her. Once she keeps going through the threads of memories, she's able to reach different places that she wasn't able to, maybe because of Namine's help. Through this process of finding Sora, she's accessing her own memories that was locked away and since this may all be happening during her year of sleep, she somehow was able to enter The Final World.
In the trailer, it looks like she's been there in MoM at least twice. Once with Riku, talking to multiple stars, and then her facing off against the man in the black coat (and she seems to be alone). Chirithy did say that sleep and death are close together, so it's possible that she's dived into her memories while in her Heart (which is a Sleeping World, technically) and accidentally ended up there. Her being with Riku in The Final World could be the second time this happens to her because now she knows how to get there, plus she'll remember this location due to Sora bringing her there in ReMind.
So I think Namine's gonna tie a lot of this together, because she was able to "trace the connection" and get the Lingering Will to help the Guardians of Light not get murked a second time (which is actually insane when you think about it). If she can turn the tides that drastically, I don't think (or at least, I HOPE) she's going to completely benched anymore. Before, it was understandable because Namine wasn't her own existing entity, so you can't just go up and ask her to help on anything. Now she's her own person, so that opens up a lot of doors.
