My favorite Water-type starter is Blastoise. Maybe it's first exposure bias, but none of the other Water-type starters come close to how cool Blastoise is in the design department. It's iconic! Blastoise is actually my favorite fully-evolved starter, including both Grass and Fire as well, when it comes to design.
That said, it's not perfect for me. Its stat layout isn't the best and its Hidden Ability could be better. Its Mega Evolution fixes these issues, and I like how it looks too, but considering Mega Evolution itself is a generational gimmick that we can probably only expect to see sporadically, if ever, after Legends: Z-A with its focus on the Kalos region.
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My thoughts on the others:
- Feraligatr: I was pretty fond of Feraligatr for a while. Crystal was my favorite game for years due to the addition of color to the series, the ability to choose a female trainer for the first time, the sprite animation upon encountering a Pokémon, and just Johto in general. I quit the series due to peer pressure and missed Generations 3 and half of 4 until a friend got me interested again by making me aware of HeartGold and SoulSilver. Anyway, Feraligatr was my starter in Crystal and it was great. As I got back into the series, Feraligatr has sort of fallen in with the pack and gotten lost a little bit. If playing a Johto game, it's usually a choice between Feraligatr and Typhlosion but I heavily favor Typhlosion now. I likes it Sheer Force Hidden Ability. Ultimately, as things stand currently, Feraligatr just feels kind of middling? I still like it, but it's easy to look past.
- Swampert: Eh. The Hoenn starters just don't appeal to me that much. I don't dislike it, but it's never going to be my first choice. Sorry Mudkip.
- Empoleon: At least in theory, I should like Empoleon more than I actually do. Penguins are my favorite animals, Empoleon's design is really neat with that trident crest on its head, and its Water/Steel dual-typing is really nice. The thing that ruins it for me, though, is its Speed. It also had Defiant as its Hidden Ability for numerous generations, which wasn't really very useful given its stat preference for Special Attacking, until Scarlet & Violet finally changed it to Competitive. That change makes me feel like it might be due another look with fresh eyes for me to judge it nowadays.
- Samurott: The samurai concept is neat, but ultimately Samurott just isn't my thing. I prefer its pre-evolutions Oshawott and Dewott and sometimes like to just keep Dewott from fully evolving. Its Hisuian Form is so similar that it almost feels pointless, though Samurott does fit the setting well anyway, so fair enough I guess.
- Greninja: Until the nerfing of the Protean ability in Scarlet/Violet, Greninja was the best Water starter to actually use and probably still is. Its ninja design and theming is pretty neat, its stats are what I'm looking for generally. I'd have to say it's solidly on the higher end of the Water-type starters for me.
- Primarina: Primarina's just an all-around solid Pokémon for me. The design is very pretty and majestic and its singer/entertainer concept is neat. The Water/Fairy dual-typing fits its concept really well. Its Special Attack and Special Defense carry it, as the rest of its stats leave a bit to be desired, especially its Speed (but that's Alolan starters for you) which is exactly just as bad as Emopleon's. Primarina makes me wish that they'd get rid of the uneven gender balance for starter Pokémon and just make them 50/50.
- Inteleon: Honestly just kind of looks like a furry art lizard character, but I like the sniper design concept and it has nice stats. The Gigantamax form with it perched high on a vantage point with the rifle is cool.
- Quaquaval: I like birds and the design isn't bad. Not too crazy about the Quaxly's evolutionary line going down a dancer route but, I mean, it could be worse. The Water/Fighting dual-typing fits its gimmick. Not bad, but a bit of an odd duck.
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Ranked:
Blastoise > Primarina > Greninja > Empoleon > Inteleon > Feraligatr > Quaquaval > Samurott > Swampert