I feel like based solely on design, Dewgong needs it the most. It is so boring ... I'm sorry.
Based on popularity and legacy, Dragonite feels like a no brainer. And I think it would be interesting to take advantage of how it looks so different from its pre evolutions.
Parasect would also be interesting for reasons UF8 and Andy have said. I think this would be my top personal pick for a Pokemon to get extra attention.