gangplank is the most viable but not as an ad carry. he's more viable cause he's a good tanky dps bruiser/peeler. probably a better jungler or top laner (his global pressure is good when he gets his ult) plus it helps in team fights cause it reveals the areas so you dont have to worry about possibly losing vision of anything.
however, in the traditional "phantom dancer/ie" type of ad carry i'd say yi and tryn are probably the closest to being viable. since yi has a good gap closer and with some AP at the start they both end up getting good heals with just a little bit of ap from runes. and late game they can both be monsters.