Let's assume for some reason that Gamefreak implements a system that saves a player's 1st hall of fame team into a database, and from that database, they release usage stats showing which Pokémon were most commonly used to beat the game with.
Of course, given the easy difficulty of all Pokémon title, I don't think there's much of a statement to make on viability. I'm more interested about the correlation between rarity & how much a Pokémon is used, among other factors that determine whether or not a Pokémon ends up on a team or not.
With such stats at our disposal, which fully evolved Pokémon do you think would be the least used for each Generation/game?
Do consider, for older generations, the likes of HM Slave, among other aspects that may be important. Try to look at the bigger picture overall.