I agree with everyone else who's said Lillie is probably the most popular classmate. I think it's a combination of her being a prominent game character, having interesting backstory potential involving her fear, and many people considering her the "pokegirl" of the saga, moreso than Mallow and Lana.

For me, the most interesting classmates are Kiawe and Lana. I really like all of the mentions of Kiawe's family (especially his connection with his grandfather), home life, dedication to training and Z-Moves, and his reactions to certain things and situations. I think Lana is interesting because she's the first female in the cast to have a Z-Ring, and because although her personality comes across as more quiet and calm, she's been shown a lot to get involved in battles, which again is something I feel like has been slacked on when it comes to female characters.