I was thinking weirdly enough for KH3, it wouldn't be a happy memory but a sad one...one of Sora on his hands and knees crying. After all, that would have been the last moment Riku saw Sora in that specific timeline before Sora goes back and changes time.View attachment 13060
I think this would be a good moment for KH1, seeing Sora is just genuinely happy to see Riku. Even, if Riku at the time couldn't recognize it as such, I do feel like Current day Riku would appreciate it. Though if we were to match it with Kairi's being the end I guess it'd be him and Sora parting ways at the door to darkness.
As for KH2 I think your #5 would be my pick, seeing it is a memory of Sora caring for him, reaching out and supporting him.
I'm drawing a blank with KH3, my gut feeling would be Sora's departure to save Kairi. Or when he just randomly appeared to block Aqua's attack that might also be a good pick.
Now I can't stop thinking about how Riku would totally have all of the sad heartbreaking moments or ones where Sora is crying...KH1 would be the door, CoM would be Sora sleeping in the egg, KH2 would be Sora reuniting with Riku...KH3 would be the Keyblade Graveyard.
But yeah, if it was KH3, it would be the one from the ending. It's the same image Riku uses for his Kingstagram post where has hope in Sora.