As for me, mine are:
Sandshrew line, Lapras, and Cubone line (Kanto)
Cyndaquil line, Mareep line, and Wooper line (Johto)
Mudkip line, Metagross line, Aggron line, and Flygon line (Hoenn)
Turtwig line (best starter!), Piplup line, and the Gastrodon line (Sinnoh)
Oshawott line, Roggenrola line, Scolipede line, Reuniclus line, Litwick line, and the Golurk line (Unova)
Espurr line, Flabebe + Floette (not Florges), and the Chespin line (Kalos)
Rowlet line, Lurantis line, Marshadow, Poipole line, and the Alolan forms of the Sandshrew line (ice steel is such a cool typing!) and Marowak (fire stick!) (Alola)
The Meltan line is also very cute, Gen 7 but technically not Alola
Falinks, Corviknight line, Snom, and Copperajah line (Galar)
Best legendary is definitely Kyogre, because whales are cool. Marshadow is second though. Volcarona isn’t legendary but would be the top pick if it was. Best starter is Turtwig, followed by Mudkip and Rowlet.
If you’re not sure what your favorites are (given there are so many), there are some cool tools to help decide.
https://www.dragonflycave.com/favorite.html in particular, lets you sort through big groups of them to narrow it down. If you’re consistent about transitive preferences then it should be right, but of course it’s all too subjective to be consistent.
https://cajunavenger.github.io/ is another neat one that encourages you to choose favorites by gen and type, then pick from those winners. Also a fun way to do it, but then some things that are too similar can “knock out” one another too early.
I just did tier lists for each region and found my favourites are usually at the top of those