7. Lowest Rank Mansion - Luigi's Mansion
Now I know you might argue that the worst ending from Luigi's Mansion might not count as a "Secret" ending as you literally only have to end the game with a terrible score, but I just want to ask, have you actually TRIED to get this ending?
It's borderline impossible!
In order to net you a shack that not even Groundskeeper Willy would want to stay in, you have to actively avoid any and all cash collectibles throughout the game. When you consider how prevalent these piles are and how aiming your super suck of a weapon at ANYTHING usually results in coins exploding all over the place, this is a terrible ending that takes pure restraint to unlock!
You have to be so accurate to not hoover up anything that you actually need to be pretty decent at the game, making for a terrible ending only pros can hope to see.