If you want to know who's fun to fight against and who isn't, here's a list in the following categories:
Fun to Fight
Mega Pony: His kit is primarily set up in a way similar to mega man games, he has no true rage function just his various modes that each have ammo limits.
Pip Squeak: He's kind of fun in a way, but his main rage isn't fun for the least part since it's ruins a lot of the other team's movement options cause underwater is annoying, but he's fun to fight against if you have that one person constantly summoning Minions instead and have them go Pirate raiding, heh. (Which is almost 95% rare to see someone do this).
Sweetie Bot: She's kind of fun to fight since her rage is similar to Princess Twilight's R rage, but rather than that she's almost as good as a doing a no rage run since her rage can backfire sometimes.
Fluttershy: She's mainly focused around being a medic that needs bears to help her out, so relying on forest friends is kind of fun rather than the fact that the bears do unnecessary damage, but hey at least she's much more flexible than her previous version.
Not Fun to Fight
Every other boss that wasn't listed in the above list, since most of them either have one of the following:
Stun and or Fear rage Hales.
High chances of random criticals for no reason at all.
Demo Hales that have eyelander weapons for no reason at all.
Not fun mechanics like Flutterbat, Eapplejack, Applejack (Mainly this Hale, right here)
Spike: why does he have airblast... like seriously.
Pinkie Pie and Braeburn: Players that keep trying to exploit the R rage in air business.
Sea Pony: This hale is just no, i'm pretty sure it was amazing in paper, but in-game it's bogus to fight.
Nightmare Moon is still very unfun to fight even after getting a nerf from the 500% dmg bonus to 400% and "no random crits".
Rainbow Dash: This hale is just super annoying to fight considering she breaks all knock-back she takes by just doing her double/triple jump which negates it, plus she can super jump and then double jump in mid-air to be able to weigh-down when most other hales can weigh-down when they super jump, so in general this Hale is just a huge mess.
All spy hales damage on this server
Celestia and Luna: both their rages are annoying to fight against, well mainly Celestia's since she can spam her rage without any cap at all.