Pokémon: Let's Go, Pikachu! and Let's Go, Eevee! was actually the first main series Pokémon game I ever played and it give me my first experience as a Pokémon Trainer! I enjoy playing the games with some of the catching mechanics from to Pokémon Go! Also, I like the Mega Evolution that put in the Let's Go games, which Mega Evolution was first introduced in the Generation VI games Pokémon X and Y.
The Partner Pokémon, Pikachu and Eevee are also my favorite starters of all time because they have exclusive moves and Secret Techniques that the normal Pikachu and Eevee don't! Plus they stay outside of Poke Ball and ride with either on your head or shoulder!
Plus they even introduced the Mythical Pokémon Meltan and its evolution Melmetal, which they were first discovered in Pokémon Go, which cause Meltan and Melmetal to become my favorite Mythical Pokémon!
Finally, the main reason why I choose Pokémon: Let's Go, Pikachu! and Let's Go, Eevee! as my first Pokémon Core Series Game is because I wanted to start my Pokémon Journey with my partner Pikachu in Pallet Town in the Kanto region, the same as Ash Ketchum, the main character of the Pokémon anime and his partner Pikachu of the very first episode of the Pokémon anime!
I really enjoy Pokémon: Let's Go, Pikachu! and Let's Go, Eevee! and I hope they make another Let's Go games in the future, along with other Pokémon Games!