Most Favorite: Yoshi. He's green, which is the best color. He's cute, but there's still room in the protrayal to have him be fierce. He has lots of different abilities. In particular, his eating abilties are funny, especially when used against large things and/or serious characters. There's a certain "security" one feels when they have the Flutter Jump available to course correct their jumps. He's my secondary main in Smash Bros. because there are many cases I want more bulk than Kirby but more speed than King Dedede, and is my sole choice for the Spirit Board in Super Smash Bros. Ultimate because of this middle ground he has. Yoshi is the only Mario & Sonic Mario-alligned non-Daisy Speed character, meaning there's no other (palatable to an immature boy who wouldn't want to play as a girl character) Mario-alligned character capable of going up against Sonic, Shadow, and Metal Sonic in races. Because of that, I have fond memories competing as and against Yoshi. Oh, and last one but Poochy & Yoshi's Woolly World is a good game.
Least Favorite: Pi'illodium and Pi'illodium X. A real bully of a boss fight. Its armed to the teeth with attacks that are difficult to track and counter, like a scattering laser barrage that requires both quick recognition and solid memory to evade. I have neither, so I am forced to mash jumps and hope for the best. Then it has a chasing attack that really challenges the player's depth perception to land the counter by jumping on it when it rushes in, but Dream Team uses sprites for characters so depth perception gets funky and the game punishes the player pretty hard for missing the timing and getting hit by it. Then there's the offscreen hammer swing, an attack that comes out so fast that speedruns of Dream Team use HOME Menu buffering to reliably counter it. If its that fast, what chance does the casual player have to counter it? All topped off by the boss having healing capabilities from Mini Pi'illoids / Mini Pi'illoids X.
Just being a hard boss wouldn't be enough to get the least favorite slot, and neither is being a minor character that only serves as a boss battle, but Pi'illodium gets the slot for two reasons. The first is that Pi'illodium X has a reasonable chance of ending a Battle Medley run through either actually beating the player or wearing them down enough that Antasma X can finish the job. The boss's "real time self destruct sequence" can trick the player into recklessly attacking to make it stop. Players should be taking the time during that sequence to heal before defeating Pi'illodium X, because if the player enters the Antasma X fight at low health and Antasma X outspeeds them, Antasma X may very well end the player's Battle Medley run before the player gets a turn in the Antasma X fight to heal. This also means the Pi'illodium X battle is tasking the player with executing menuing quickly in order to fit both healing and draining the boss's HP within the same time window, which is difficult to do while making informed HP and BP decisions that will echo into the two remaining fights of the Battle Medley. Second, its existence suggests something interesting about Pi'illo culture that the game doesn't take the time to dig into. Prince Dreambert can say "Pi'illodium is a guardian" all he wants, but Pi'illodium's abilities contradict this. Civiliations don't build weapons of war that powerful without battlefield experience. It can fly across long distances to pursue targets even though its purpose is ostensibly to stay in one place to protect it. Something intended to protect a sacred place like the Pi'illo Temple shouldn't have a self destruct system that could very well destroy that sacred place. Something's not right with Pi'illodium, and by proxy the Pi'illo Kingdom, but Super Mario stories are simple by design so Dream Team doesn't examine this. If there's anything I don't like, its a missed opportunity.