For some time, Pokemon has had an issue with post-game. They still haven't brought back the Battle Tower (damn you, Game Freak), and while they've done a lot of quality of life fixes like Hyper Training and Nature Mints, there has been a distinct lack of content.
Indigo Disk definitely helps to ease the pain. Blueberry Academy feels lively and full of interesting things to do, including the ability to catch nearly EVERY start Pokemon from previous generations, which is... a very big deal, especially for shiny hunters. Every trainer battle within the DLC is a double battle, which is the current competitive format in Pokemon, further allowing newcomers to sink their teeth into the competitive scene easier. It introduces a system to catch most legendaries as well, and more interactions with characters from the main game. There's even a GACHA MACHINE.
It's truly jam-packed with stuff to do, and while the story is a little longer than Teal Mask, and the map is a little bigger, there doesn't feel like there's a whole lot to do in the main story for it. The post-game however, feels good. Finally, there is longevity to my Pokemon game. It's made me play consistently again, something I haven't done for some time!
Add on top of that another fantastic legendary design, the Stellar tera type, the Synchro mechanic, and the resolution to a certain character's crisis that was set up in Teal Mask, and Indigo Disk is up there as maybe one of my favorite DLC for any video game, not just a Pokemon game. Honestly just a blast front to back, and it made me realize how much I like using my Switch again.

Pokemon Scarlet and Violet are some of the best written stories in the entire Pokemon franchise, bar none. Starting with Arceus and then into the four main plotlines that make up SV, it truly feels like it's in a story renaissance (albeit at the sacrifice of some performance issues).
The Hidden Treasure of Area Zero cranks that story up to 11. Teal Mask introduces a pair of delightful characters whose dynamics are fun and inviting and who are fascinating to see operating outside of SV has you understand of Paldea. Kitakami is an absolute delight to explore, if a bit small, and the small-town rural Japan vibe is absolutely unmatched (especially since it allows for Hisuian Pokemon to show up!).
The box legendary (Ogerpon) is maybe my favorite legendary in the past few years, even if the three sub-legendaries are... lacking.
Seeing even a scrap of Teal Mask convinced me to pick up the DLC and I did not look back... especially after Part 2.