The Summer update Wave 1 or the first update have been probably my favorites, mostly because they added new gameplay loops (diving, collecting art, bushes, etc.) but every update so far has brought something to like. The mentioned summer update was specially helpful by fixing the visitor schedule, the best QoL update this game truly needed.
For reference, this is a small summary of everything that has been added: (art and bushes aside as you experimented them)
* 117 furniture items
Obviously you'll have some people who can't find anything they like, but chances are you'll like something out of the 100+ items added. Most of these feature some kind of extra element: source of lighting (like the Turkey Day decorations), trigger events for the islanders (like the Festivale stage), interact with the wind (like the Festivale Balloon Lamp), have little animations (like the Puppy Plushies), act as surfaces (like the Turkey Day garden stand) or in a single case, add a new fast-travel mechanic (the Pipe).
* 42 interior items
For your home interior design needs, many different-shaped rugs are added like the Shamrock Rug or the Pirate Rug. Wallpapers like the Mermaid one glows/looks differently without light as well and the Mushroom Mural is animated.
* 84 clothing items
If you needed even more clothing options, some backpacks and many "dress" options have been added. King Tut Mask is a notable one, as that brings back tripping.
* 15 tool items
Most of them bringing new animations to the table, like the Shamrock Soda or Sparkling Cider having a "cheers" animation, the Megaphones (not the NL one) making you "cheer", the balloons have animations around them, including the sad one of seeing them fly away when you trip and fall.
* 11 miscellanous items
Things that are mostly used as crafting materials or the like, but can be used for decoration or worn, like the feathers from Festivale or the little pearls from Pascal.
* 40 sea creatures (and Diving/swimming)
Returning mechanic with differences from previous incarnation. Source of a new "daily" with Pascal and well, the sea creatures. They bring more designs to the table and many animations, like the Mantis Shrimp punching the glass tank it's trapped or the Horseshoe Crab and Spider Crab being "placeable" like the Snapping Turtle- they both feature animations as well.
* Pumpkin farming
A new mechanic, a mix between the flower and fruit system. It requires watering to get the better results, it has a random element planting from the Starts that can be avoided by planting color directly, etc. Many are hoping to see this expanded into a full farming mechanic with more vegetables.
* Character customisation options expanded (6 new hairstyles, 4 body paint colors and 6 eye colors)
Notable as the new hairstyles are mostly aimed at POC. The latter two are geared towards Halloween, but they expand your options to cosplay/present yourself.
* 15 reactions
Some are a bit more elaborate than standard reactions like the Confetti which interacts with the wind or the long Yoga one and all NPC and islanders got them and can react/perform them as well.
* Six new/returning NPC [not including major events NPC] (Pascal/Gullivarrr/Cyrus/Reese/Rover/Luna)
* 4 major events (Halloween/Turkey Day/Toy Day/Festivale)
* 5 minor events (May Day/Museum Day/Wedding Season/Fireworks/Valentine's)
* Some tiny events either acknowledged a bit like Chinese New Year/Nature Day or through the seasonal tab
Lumping all these together, as they are limited time things. Notable things are the new events and adjustments to previous ones, like how Valentine's isn't gender-locked anymore.
* Dreams (and subsequent improvements like previously visited dreams and random dreams)
* QoL to the Custom Designs machine search feature
* QoL for the visitor schedule system
* Other tiny QoL improvements like the stereos outside
* Expanded storage (800 purchasable slots)
* Catalog app on the NSO app
* Save and transfer feature
Some of the QoL updates some people insist have not happened at all. YMMV on the usefulness of them, depending on your needs. As said, the fixed visitor schedule is easily the best one for me. Expanded storage was a gift from Arceus itself, 800 more storage slots are great and they also gave a better incentive to pay your last home loan. The catalog app is the third best for me, it's very helpful to be able to check your catalog while still doing things on the game, instead of pausing each time to check your NookPhone. Fortunately, I haven't had the need to use the backup save or transfer features, but they are inherently useful.
I'm probably forgetting some minor details, but thats's basically what has been added. Of course, you can only want specific things and if those are not here, you can be disappointed-