Has No One Replied? I'd Be Honored to be the First!
My favorite minigame would be Haunted Hallways, But it was close with Roller Revenge and Splat-a-Stamp.
For Coinathlon, I agree with you 100%. Leafboard Hoard is fantastic. I wish it was made into a normal minigame, where whoever collects the most coins (Everyone is in the same tube) by the end, whoever collected the most coins wins! (The coins would be scattered) Or it could be a coin-collecting game with the same formula, or the new one I just was talking about.
For Bosses, King Boo and put your hands down. Easily the most challenging boss in my opinion, and while simple, it's very fun, even outside of Toad Scramble, which some of the other bosses *Cough Kamek Cough* can't achieve this as well as King Boo does. (Also yes, I HIGHLY DISLIKE Kamek's fight, in both modes.)