Since I seen a lot of people getting on about RR over the Looker quest.
I got to say it, I'm not a fan of the Looker Quest in SM for a couples things.
1. The only reason this was a thing is because the main story of SM completely ignored the Ultra Beasts Lusamine let loose and instead hyper focused on Lillie and her family drama. The Ultra Beasts, never got to do anything impactful which leads to.
2. The Ultra Beast quest was boring, you go talk to Looker, get told about your UB and you go catch it in a random route. Rinse and repeat. Unlike in Ultra which has you explore the Beasts world and see how those things live and thus get to know more about them. This is a pretty underwhelming use of them.
3. Plot point, the entire Aether Foundation got off scott free for helping Lusamine directly threatening Alola, Interpol was doing shadey dealings with them still and Gladion was left for the wolves having to monitor that. A karma houdini and a character who WANTED to leave that place gets screwed over. Pretty lousy ending.
In Ultra instead, those guys were legitimately trying to protect the world, but their boss was an egotistical Leeroy Jenkins that made things worse and escalated the problem, them being allowed to continue work is a little easier to stomach since their boss wasn't directly/ intentionally screwing things up and she got her head out of her butt.
Also, Gladion doesn't have to get chained up in that place.
4. Most of the Looker plot has Anabel be taken out of the front lines and leaving everything to you, it's taking a lot of agency away from Anabel (A frontier brain) and this keeps going on. It isn't exactly comfortable and the only thing we have to gain is knowledge about Fallers that becomes relevant in Ultra (Mohn closure) and SwSh (Max Raid adventures.).
A good post game event, this is not.
In contrast, Rainbow Rocket IS a better post game event, in both a narrative stand point and in general.
1. In RR, it is all about a bunch of crooks from alternate realities trying to invade ours and it's up to you to kick their butt. Most of them act a bit alien because they were yanked from the reality where they won. Even if it was confined into once place, this results in one of the longest dungeons we fought against, that dungeon having puzzles and a good challenge from some of the bosses. (Looking at Lysandre and Giovanni.) Most of the boss's actions fit with what we know of the characters. (Archie and Maxie getting at each others faces, Lysandre being a mad man, Ghetsis being the most evil of the villains and Giovanni being the only one who has both brains and sanity.) or having things actually expanded upon. (Cyrus regretting everything, his interactions with Rotom and how this bit of his behavior is seen in Masters of all places.)
It's a lot more than nostalgia pandering, it's a proper 20th anniversary event for what should have been the 20th anniversary game and it makes uses of the concept of alternate realities that Gen 7 was putting more focus on.
2. RR had a lot more in delivering pay off for certain characters we know of and that makes up for the over the top nature of RR.
Lillie as a trainer and standing up to these guys, standing up to Faba and standing up to Ghetsis of all people. (Compared to SM where she went crawling back to her mother and becoming a trainer off screen entirely)
Having Guzma do something heroic and showing us his redemption. Colress's moment of booting Ghetsis from our reality and showing Faba as the sniveling, dubious creep he has always been and showing us a bit of his demotion.
Compared to SM where we were only hinted at with Guzma (like how SM pulled that hint at Lusamine turning around... completely out of nowhere) Colress being just a cameo and us only being told of stuff regarding Faba.
To put this into notion, RR made Festival Plaza relevant in the active story, rather than a... player feature for us to deal with.
3. After RR, we start getting our closure for certain characters if we go around to look. Lillie stays with Kukui as his assistant, yes, but it is implied that she is exploring Alola instead or is going to (Masters points this out since it is flat out stated that she stayed on Alola.) and is in a more healthier environment. We also have Lillie fight alongside us at the Battle Tree as an activity. We get more on Lusamine before RR where she was having an identity crisis and after where she vowed to keep her distant from her kids, along with turning the Aether Foundation around for the better. (As well as doing some self reflecting when we fight her at the league.) We see Guzma vowing to better protect his homies of Team Skull and meet him as a title defense challenger.
Gladion comes back to Alola a month later after beating RR with some confidence in his own strength and we meet Mohn at the Aether Foundation to see that closure to the family drama.
For those complaining that Ultra Sun and Moon's main story changed the focus on the Aether Family drama for you the player and Necrozma, THIS is where the Aether Family got some of that focus back and in proper closure (Masters taken the Aether Family from Ultra's outcome and that is where more confusion should have been cleared.) and I'm glad we got that.
Ultra's post game was better than SM's and is better than base SwSh's. (The expansion packs are something else.) It is one of the better post games we gotten, right next to BW2's (BW2 has better events) and Gen 2's (Longer and a whole region to explore.)