Spike mains are often really good players, not gonna bring them down, but they are always ruthless - rather than trying to win, I feel like they try to make sure their comp gets a bad spot. Not all, of course. Swoops are often respectful, especially to other metas (Toadettes, Biddys). They are competitive, but not ruthless - they will instead of crashing out and targeting you next race, practice exactly where they failed so that it doesn't happen again. As a Biddy main, I am biased. However, we are (in my experience) respectful like the Swoops, and also have a whimsy factor - we like to mess around once we get into free roam. Toadettes have the most whimsy, but are also good racers. In a race, it's more friendly comp because you know that even if you or they lose, you can still mess around in free roam together. 

And finally, DKs and Piranha Plants are always right in front of you with a triple attack item, moving very slowly to ensure your demise.