actually giving explanations now but only for the top 3
i put Sinnoh first because it has one of the best champions (Cynthia), my favorite legendary trio (Palkia, Dialga, and Giratina), it has Bidoof the legend, and it has a bunch of other pokemon that i like (Weavile, Torterra, Mismagius, Honchkrow, Floatzel, Luxray, etc.) + Legends Arceus was an AMAZING game. And the story is cool (i forgot half of it lmao)
Hoenn is second because it has the best starter trio (Treecko, Torchic, and Mudkip by beloveds), has a lot of cool legendaries (Groudon, Kyogre, Rayquaza, Deoxys, Latias and Latios), and also has some other nice pokemon (Altaria, Breloom, Salamence, Metagross, Flygon, Armaldo, Aggron etc.) Steven Stone is also a cool champion
Unova is third because Reshiram and Zekrom are cool legendaries (plus the Swords of Justice and Victini), it has a very good story, and introduced some cool pokemon (Krookodile, Haxorus, Druddigon, Stoutland, Excadrill, Gigalith, Zoroark, Chandelure, etc.). The starters are kinda mid though, Oshawott is the only one i really like-