Goldeen and Seaking
Now this is a forgotten one. This big, colorful koi is basically just kinda there, a pokemon few people feel strongly about either positively or negatively, but there's nothing wrong with that. I like that some pokemon are just plain old birds and fish, I like the depth it adds to the setting and I'm delighted when I run across somebody whose absolute favorite is such a humble creature. There's somebody out there to whom Goldeen must be deeply special and that just makes me smile.
The same is undoubtedly true of Seaking, though the two are different enough that you're likely to prefer one over the other. Personally, I am a bigger fan of Seaking's round, glassy fish-eyes, and I kind of only just noticed how it has little vampire fangs protruding from its otherwise weirdly human, pink lips. I must have seen that thousands of times in my life and never really stopped to appreciate it until today. Seaking really is majestic!
These are some super alright fish!