Grass is my favorite of all of the 18 Pokémon types, so that extends to the starters as well.
It's not really a hard rule when it comes to starters since I tend to play different versions and choose different starters for different runs, so I use all of them eventually.
I'd say Fire is second in my preference because Fire tends to be a less common type in the wild. That leaves Water in third, but that said there are some Water starters I love too. Water admittedly feels the least appealing generally because Water-types are so common in the wild.
Order of Preference by Generation
- Kanto: Blastoise, Venusaur, Charizard
Blastoise is my favorite final form starter ever. I like the concept and versatility of Venusaur. Charizard is cool but overutilized.
- Johto: Feraligatr, Typhlosion, Meganium
Feraligatr and Typhlosion are pretty even and I alternate between the two in Johto runs, though I have a slight preference for Feraligatr generally. Meganium is pretty but I don't like the defensive stats and moveset for an in-game playthrough generally.
- Hoenn: Blaziken, Marshtomp, Sceptile
Blaziken, with or without Speed Boost, is the best of the three for me. Pretty even between Marshtomp and Sceptile, they're both good in their own ways.
- Sinnoh: Torterra, Infernape, Empoleon
Torterra is my favorite due to the cool design with the tree growing on its back, and I like the Grass/Ground typing. They're the trio I like most evenly, they're all close. I love Empoleon since penguins are my favorite animal; Infernape only edged it out because Infernape's faster and has Iron Fist as its Hidden Ability.
- Unova: Serperior, Emboar, Samurott
Serperior is far and away my favorite of the three, I love the Snivy line a lot. I like Emboar's design even despite the redundant Fire/Fighting typing. Samurott is cool but I prefer its pre-evolutions.
- Kalos: Greninja, Chesnaught, Delphox
Protean Greninja is too good not to be first here. Chesnaught edges out Delphox because I love knight aesthetics, but Delphox is cool too. Strong trio of starters.
- Alola: Decidueye, Primarina, Incineroar
Rowlet is my favorite first form starter ever and Decidueye being a Grass/Ghost archer owl is awesome. Primarina has a pleasant design and concept. As for Incineroar, I like it since I have been an on again off again viewer of professional wrestling throughout my life and I like the heel wrestler concept, but honestly, I prefer Litten and Torracat over Incineroar. I even have a team planned where I'll leave Torracat as an unevolved middle stage.
- Galar: Rillaboom, Inteleon, Cinderace
I'm not crazy about its preevvolutions, but Rillaboom is awesome. Cinderace and Inteleon can switch places with each other depending on the day, they're both good.
- Hisui: H-Decidueye, H-Typhlosion, H-Samurott
It took a little bit to warm up to, but I grew to really love Hisuian Decidueye's design and I appreciate that its coloration is autumnal. Hisuian Typhlosion has got a bit of a goofy face but I love the Fire/Ghost typing and the purple flames. Hisuian Samurott, while thematically appropriate, feels uninspired and boring.
- Paldea: Meowscarada, Skeledirge, Quaquaval
Meowscarada and Skeledirge are pretty even for me. While I'm among the people who wish the line would've stayed quadripedal to some extent, I like what we got with both Floragato and Meowscarada in the end. I love Skeledirge's design and concept, it's Fire/Ghost typing, and I think the middle stage Crocalor is cool too with its calavera face, fire/marigold flower/egg sombrero and its little vest pattern. Quaquaval is third by process of elimination but I really like the design. Very strong trio of starters in my opinion.