Definitely P3's Hermes for me, but there's also P2's Vulcanus and Apollo, P4's Izanagi and Konohana-Sakuya.
Any other design I like is technically demons from SMT, so really these are the stuff for me.
Arsene. He's stylish and looks sinister. He's the coolest looking persona in my opinion because of all the style he has. And in my opinion akira and arsene are the greatest duo of persona and persona user. They just have so much style and flare together.