Dorayaki
King Bowser
*To define bosses: we go with broad definition, any character that lead an entire country, political region, army or even a corporation. Not just villains but positive characters count too.
I find it weird when it comes to big sport contests in Mario world, we don't really see a bunch of lords or bosses compete for a race. Like, usually a country wants to nominate it's most talented candidate, most rulers or leaders happen to be more capable of the sport thing. So it feels weird when the countries don't try to push strong candidates for the honor.
Yes, in series history we've got crazy villains like Shadow Queen who just devotes to world end. But we still got many peaceful leaders like King Bob-omb or Prince Mallow, or ones like King K.Rool who'd seek for a violent competiton like Bowser does. It feels like most of them wouldn't really ignore it on purpose. In fact, it'd be a really fun idea if Peach uses the sport contests as a kind of diplomatic activities.
I get that some of the party subjects exist for pure fun, so it doesn't matter if you're a nameless minion or citizen. But in several Mario subjects the rosters also require diversity in weight and power, so I find it weird to not put more XXL size characters in the use of diversity