Depends on what you mean by "favourite".
If it's my favourite fish to eat, it's salmon, cuz sushi is da best.
If you're talking about my favourite fish as a pet, I'd say goldfish. I had one as a kid that died (surprise!) but I liked him and thought he was kinda cool. I named it "shiny scales", because I was just creative like that.
But If you're trying to ask what's my favourite fish aesthetic/appearance wise, I think that Kois are stunningly beautiful. I want a pond full of them in my house and it would look great with lily pads and algae, and I think that a small waterfall would be amazing.
What's your favourite insect? (Since we're in the topic of favourite animals)