My favorite Pokémon from each generation are as follows:
Generation I: Starmie - Great Pokemon, beautiful, and she was appealing in the anime when Misty used her. This is also my all-time favorite Pokemon. (Favorite legendary of this generation is Zapdos because it was the only Electric-type to bypass Ground-type moves in this generation).
Generation II: Tyranitar - Very strong and tough looking. It was worth the leveling up to get. (My favorite legendary was Lugia, mainly because of the second Pokémon movie, which I enjoyed).
Generation III: Gardevoir - I love her because she is very elegant and is a strong special attacker. Blaziken is an extremely close second. (My favorite legendary is Kyorge because I love strong Water-types).
Generation IV: Garchomp - This one was extremely tough because I love many Pokémon from this generation such as Weavile, Electivire, Mismagius, Magnezone, Togekiss, Honchkrow, and so on, but Garchomp has helped me win the most battles, so I guess he gets a slight bump to first place. (My favorite legendary Pokémon this generation was Dialga because of its Dragon/Steel typing, which makes it super tough).
Generation V: Haxorus - I loved A LOT of Pokémon from this generation, but I loved Iris's Axew from the anime, which made me have to get one for myself, and I love Haxorus's high attack stat, which is perfect for sweeping. (My favorite legendary of this era was Reshiram because it is beautiful, has a great cry, great stats, represents the truth, has great lore behind it, and I can go on and on. It is also my all-time favorite legendary).
Generation VI - Sylveon - Very cute new Eeveelution that represented the new Fairy-type, which I love. Greninja was also a very close second. (My favorite legendary was Yvetal because it was just so dark and destructive, which is something different for Pokémon).
Generation VII - Decidiueye - Very cool grassy, ghostly owl archer. I love it. Primirana is also very close second because she is beautiful and elegant like Gardevoir. (My favorite legendary was Lunala from Ultra Moon because it looked so cool and creepy. Not sure if Ultra Beasts count as legendaries, but of all them, I loved Buzzwole because that design is incredible).
Generation VIII - I am currently new to this generation, as I only got Pokémon Sword for 2021 Christmas, but so far, my starter Pokémon Inteleon seems to be my favorite for now. I love that animation for Snipe Shot and all the critical hits I land. (I haven't reach the point in the game where I got to experience any generation 8 legendaries, but I will update when I do).