Not a topic I see brought up much, and is fun to discuss!
Pokémon has had several art styles across the franchise over the years. Notable/distinct styles being old Sugimori (watercolor,
click for example) and new (the typical style most official art of individual Pokémon/trainers is in nowadays,
click for example),
Dream World,
Cafe Mix, the anime's varying art styles, etc.
Do you have a favorite?