One thing I find interesting about this episode is that the Pokemon in Mewtwo's group were references to animals used in cloning research IRL.
I know many others have said this on earlier pages, but I will echo their sentiment regarding the opening which has become outdated since... the last episode, but on the flipside it is something that isn't new to Pokemon, so I'll give it pass for now. The other minor nitpick is Go's pokedex doing the cringe "great" but y'know that doesn't really matter in the grand scheme of things so it's pointless even mentioning it.
There really isn't anything else to talk about other than the Mewtwo encounter to be perfectly honest. The first half didn't have as much substance as I would like, but Go caught more 'mons and failed to catch others. Anywho, watching the episode, I got the impression that it wasn't clear whether Mewtwo remembered Ash since the enounter at Mount Quena, however we know that because of said encounter, Ash recognised the Pokemon (and it is the first time Mewtwo has appeared in the main series without the M2 Bind armour back in season 1, and yes I am aware that the armour didn't have a name in the anime, the M2 Bind in Adventures resembled the armour seen in the anime). On the other hand, I suppose I shouldn't be shocked that the writers didn't have Mewtwo recognise Ash verbally as it would then prompt the viewers to re-watch Mewtwo Returns for context.
Mewtwo did have a point though, asking a question that many of us had, in this case, asking Go how catching all other Pokemon will lead to Mew and I'll admit, Go's answer was pretty good albeit the fact that he intended on acquiring Mew in the episode.
Onto the fight itself, I can't be too shocked that Pikachu was a fall guy here, the poor thing was pretty good early on, but he's been lacking lately. In any case, I was expecting a loss here to be fair. Though to put Ash's defeat in context, this episode shows that Ash was fortunate that Giovanni had emergency matters to attend to back in EP063 because if that was not the case, he wouldn't have gotten the Earth badge considering we know that Giovanni had bested Ash in the Meloetta arc during BW, not to mention Ash's squad back then were largely NFEs who initially struggled when TR used the gym's Pokemon and now Ash has lost to the same Pokemon that beat various other trainers who challenged the Viridian Gym, including Gary, and even though Ash has gone leaps and bounds since the Indigo league, Mewtwo was simply too strong for him even now.
But if this episode is any indicator, it's that we'll likely see Mewtwo again at some point as it's acting as a mutual wall for Ash and Go to overcome to reach their respective goals.
Also, thanks to Mew we finally got to see Autumn form Deerling in the series, and not gonna lie it completely glossed my mind that we hadn't seen all four formes of Deerling, whereas Sawsbuck had all four featured in one episode in BW.
One final note, considering this is the same Mewtwo from M01, we can likely expect Dan Green to voice Mewtwo (as he voiced Mewtwo in Mewtwo Returns and Mewtwo Strikes Back Evolution) when this episode gets dubbed.