Personally, mine is joint first between Sandshrew (the cutest), Espurr (most relatable), and Gastrodon.
After those, in no particular order, I like Turtwig (best starter, fight me), the Roggenrola line (little rock guys), the Litwick line (candles and lamps with nefarious intentions), and Metagross (cool robot thing).
Kyogre is the best legendary in my opinion
Design-wise? Banette. The creepy doll is just such a gorgeous image.
In my heart? The goofy gang boss Honchcrow.
I do agree Gastrodon is cool.
I like this supermodel insect from another dimension too.