I've really enjoyed the Star Allies DLC. The game has probably some of the blandest level design since Dream Land 3, most likely to help with the drop-in drop-out multiplayer, but it also has some of the coolest movesets for copy abilities too. In particular I love Plasma's teleporting ability, zip-zapping all over the place is a blast and Plugg was probably my favorite non-dream friend to play as in guest star mode because of it. I also like the various teamwork attacks, particularly between elemental abilities like hitting Water's geyser with an ice move or electric move for a super attack, or hitting Fire's flame breath with a wind move for another super attack. It promotes further team synergy and in particular multiplayer teamwork. Getting an AI player to use these super moves requires signaling them for every single usage, but two actual players can set up one person using Fire's breath and another person constantly sweeping behind them with Clean to turn what would normally be two mid-range moves into a powerful rapid-fire long range move. Combos like this really make multiplayer feel worth it.
That said, when it comes to fantastic movesets, the DLC characters have just about everything else beat. In particular I love the animal friends and Daroach! Rick, Kine, & Coo bring with them so much fantastic support, they have almost every type of attack except electricity, ESP, a friend throw, paint, and a consistent source of ice. Plus the aforementioned wind + Fire's fire breath can be done with Rick's fire breath too so they even have a combo move of their own! And since Clean uses all of the Dream Land 3 animal friends, you basically bring the whole gang together with the combo I described above!
When it comes to Daroach, it feels like you're playing a bullet hell game, but you're the boss! So many of his moves are projectiles that have some combination of charge time, delayed activation, huge range, long duration, and high power. Filling the screen with explosions, Squeak Squad members, ice lasers, and Dark Nebula chests is a breeze and pretty much guaranteed to do a ton of damage in any boss battle regardless of how good your aim is simply because of how much stuff is happening at once. He's also probably the only Dream Friend that I feel hits a solid balance between utility and combat. Bandana Dee, Dedede, Meta Knight, Marx, and Dark Meta Knight are all really great at fighting, but that's pretty much all they bring to the table. The animal friends, Gooey, and Adeleine provide a ton of fantastic support, but they don't have the devastating offensive capabilities of Marx or Dark MK. Daroach gets a solid combination of powerful projectiles and several utilities with fire, ice, and electric elements alongside Storo crushing stakes for a more jack-of-all-trades sort of style.
Also, the unique level setups for the Dream Friends in guest star mode is a really great time! I wasn't originally expecting to enjoy playing as Dark Meta Knight all that much, seeing as I'm not a fan of the Sword ability in general. However, his moveset is almost entirely unique, and in particular I find it powerful enough to warrant the short range of a melee weapon. What really made me enjoy playing as Dark MK though was the Amazing Mirror level design duplications in his guest star mode playthrough! Most of Star Allies' level design might be bland, but bringing back the exploratory stylings of Amazing Mirror for at least a few areas at the beginning and end of the playthrough made me actively play the game and explore the world rather than mindlessly mashing my way through everything like I had done in every other playthrough! I also generally enjoyed the unique setups for using the animal friends' and Daroach's unique abilities in their playthroughs, but those were always pretty short and instances where these abilities would be helpful in normal gameplay just don't show up. Dark MK's maze segments actually lasted quite a while in comparison, making them much more exciting and giving him the best level design out of anyone I'd say, even if his abilities aren't as inherently interesting.