Ranking from most difficult to easiest to complete Pokedex:
Let's Go Pikachu & Eevee
Sword and Shield
Legends Arceus
Brilliant Diamond & Shining Pearl
Scarlet & Violet
Let's Go is the hardest because of the version exclusives. Nobody is playing this game in 2024, so the GTS requests for this game were... barren to say the least. There's also no breeding, so anything I missed when registering what I had to HOME had to be re-caught. Which was a pain. With no ability to request for specific origin marks getting what you need is not fun. While you can restrict trades to only go through if it can go into Let's Go, this also applies to GO Pokemon that have not touched a non let's go game, so do keep that in mind.
Sword & Shield is #2 for a similar reason. Older game less trading activity and needing Pokemon with that origin mark is annoying because many of these Pokemon are in Scarlet and Violet. And because you can't request for origin marks, you'll be getting far more coming from that game instead of this one.
Legends Arceus only really has 2 nasty Pokemon to get. Shaymin and Darkrai who for the first time you actually do need. And you need to buy/rent two full priced games to even unlock the missions for them. But it's straight forward from there, and the game is actually really fun to play through.
BDSP is not terrible. Only 150. And you can breed what you don't have. It's just Mespirit and Cresselia that suck.
Scarlet and Violet: tedious but that's it. You can get trade evolutions and the new evolutions with the stupid methods through raids, Pokemon appear in the overworld so you can't miss what you're looking for. The daycare being mobile helps tremendously. It's easier than ever to grind EXP candies in this game because of the Blissey raids.