Trying to decide for myself where it makes sense for the final infinity stone to be revealed. I'm leaning toward Thor: Ragnarok, just because of the setting and villain. I think my second choice is Infinity Wars I. Part of where I am coming from is where the other stones were revealed or have their focus.
-Tesseract: Now on Asgard, if memory serves. But was found on earth and that's where the action was focused.
-Ether: Presumably, the Collector still has it. When it comes right down to it, the action was really focused on earth and the invasion at the end because the merging of the realms was focused there. But obviously, there was an Asgardian focus as well.
-Loki's staff/mind stone: Earth again. Yeah, Loki brought it there from far away. But the action surrounding it is focused on earth. And that is where it now resides (in Vision's forehead).
-Orb (was it the reality stone?): Nova Corps has it.
-Time stone: Earth.
So, 3 of the 5 stones are on or were focused on earth. I just think it would be odd for the last stone to be there as well. So, that said, I am ruling out it being revealed in Black Panther or Spider Man. And in any case, I think it would be odd for it to not be revealed until AFTER the first Infinity Wars movie, so I am ruling out Captain Marvel. That leaves GotG2, Thor: Ragnarok, and Infinity Wars I. I see no reason it couldn't show up in GotG2. But it just seems odd that they would reveal 2 stones in the two GotG films. I dunno. I could be completely off base, but that's my speculation, for what it's worth.