Protean Porygon2
Greninja is currently the only viable Protean user, as Kecleon is simply garbage and has no niche in OU. That being said, I think that Porygon2 is a perfect candidate for Protean. It already has impressive natural bulk, which becomes excellent with an Eviolite, which means that Protean could potentially see some use in a defensive role, rather than an offensive one. While Porygon2 could still function well offensively with Thunderbolt and Ice Beam, the fact that it would be able to change its type to fulfill a defensive role is simple brilliant. It would enable Porygon2 to wall a different variety of Pokemon simply by changing its type as they switch in. While not a common option on Porygon2, it can run Shadow Ball, which would effectively enable it to turn itself into the Ghost-type as a check comes in, and then they wouldn't be able to destroy it with their STAB move, causing the opponent to resort to weak coverage moves.
Greninja is currently the only viable Protean user, as Kecleon is simply garbage and has no niche in OU. That being said, I think that Porygon2 is a perfect candidate for Protean. It already has impressive natural bulk, which becomes excellent with an Eviolite, which means that Protean could potentially see some use in a defensive role, rather than an offensive one. While Porygon2 could still function well offensively with Thunderbolt and Ice Beam, the fact that it would be able to change its type to fulfill a defensive role is simple brilliant. It would enable Porygon2 to wall a different variety of Pokemon simply by changing its type as they switch in. While not a common option on Porygon2, it can run Shadow Ball, which would effectively enable it to turn itself into the Ghost-type as a check comes in, and then they wouldn't be able to destroy it with their STAB move, causing the opponent to resort to weak coverage moves.