Swampert and Blastoise:
I mean I’m more of a grass starters person myself. The exception being gens 6 and 4 (but I do love Torterra, it was actually the first Pokémon I ever got to level 100) and kInD oF gens 2 and 5. Kind of for gen 2 because I don’t really like any of their final evolutions, but Chikorita and Bayleef are my favorites of the first and second stages, so it’s a hard decision (and the final evolutions are especially sad because imo Johto’s starters have the best first stages as a trio, what happened??). I mean I‘he only played through a Johto game once, and I picked Cyndaquil, but I did pick Chikorita the first time I played gold/silver.
Gen 5 is the exact opposite problem, Samurott and Serperior are both so awesome that I flip flop, I love both of their lines. I do kind of have Snivy bias from it being my first ever pokemon though.
And gen 8 I guess. Forgot that one exists. I like Intellion the most, but gen 8... lol.
If you care that much you could just look it up but fair enough.