I think I have to go into the specific flaws of Legends Arceus first, because it’s in the very core of the game and let’s go obviously doesn’t have the same problems.

The core gameplay loop of earning stars felt like a giant list of chores after awhile. If they reduced the quantity of repetitive tasks then it’d have been much better. Legends also has poor QoL features - I would’ve liked an oblivion/skyrim-esque compass ui for the open areas. Wild pokemon attacking the player is an annoyance imo. Wild pokemon attacking the player doesn’t really present a real risk of blacking out (unless you’re very, very careless) because it’s relatively easy to escape via ride pokemon, you can send out a pokemon in battle as a distraction (and win or try to move further away in the process) and there’s fast travel once you’re out of range. I don’t think there’s a good way to make this mechanic non-irritating. Increasing the difficulty/challenge would be frustrating in its own way (that’d be too punishing). The core issue is that you, the player character, don’t directly engage the enemy in combat and thus you can’t really fight back “normally”. The limited inventory system is much too harsh, and expanding it is much too costly. The background music barely plays. The combat system is really wonky and the battle order is janky especially with agile and strong. I personally don’t like the sinnoh region and hisui feels even blander. It also has, I feel, the weakest regional dex of any region. I hate the galaxy team, and there is some shitty colonial apologia in the game (I was hoping to side with the diamond/pearl clans. Also Melli did nothing wrong, colonizers can’t be trusted; I hate how he’s portrayed unsympathetically). Finally, there is a very large lack of trainer battles compared to wild battles. I feel that the former is more exciting than the latter. Oh, and there’s no boxlink (inevitable given the time period, but still)

Let’s go (or literally any other pokemon game except D/P/Pt/BD/SP being set in sinnoh) not having the unique flaws of legends mean they’re automatically better.

I think Let’s Go’s core strength is its simplicity and charms. It’s meant to be a cozy game with a cutesy/cozy atmosphere and a focus on pokemon interaction (following, riding etc). I play pokemon for the charms of pokemon (?) more than anything else, and I think Let’s Go and Heartgold are the most obviously focused on that aspect (pokemon gameplay/mechanics kinda lacking in comparison to other games in existence tbh). It was much easier for me to get attached to my let’s go team than my arceus team. I haven’t touched pokemon in some time, and I played arceus much more recently than let’s go. I still remember my let’s go team, but I can barely remember my arceus team. In terms of wild vs trainer battles, it’s the exact opposite of Legends Arceus, which suits my subjective tastes better. In addition to phys/special split and fairy typing, Let’s Go also modifies quite a few trainer teams and movesets for the better imo (in addition to adding new trainers along the routes). Kanto isn’t my favorite region but I do think it’s a big step up from Sinnoh. And the background music is a lovely remaster, and actually plays most/all the time. The core gameplay loop is more “conventional” than arceus but it is way better than the pure boredom that is arceus. Let’s go is obviously a shorter and simpler game, but it managed to make me feel happy. I didn’t really feel anything throughout the entirety of arceus.