I picked Popplio in my first UM playthrough and honestly, it was just a really fun Pokemon to use. It made Nanu's fight, which I struggled with so badly with my Incineroar in Moon, a breeze. Maybe it was because I over-leveled myself to an unreal extreme, but Primarina stuck at the head of my team the whole way through, even after I got Lunala. Bubble was a powerhouse and I have only good memories of it. It now sits in my brother's box cuz I don't know how to use Bank, and one day I'll retrieve it in my new save file.