None of the mirrors are particularly great. But i voted peacock because obvious. Imho the worst mirrors are peacock, parasoul and painwheel.
And since I'm super bored I'll explain why each mirror is terrible in my eyes:
Peacock... Both players playing rote rps games of bang x3,hk port, lvl 1 item and fake port.... Over and over and over again. The matchup can be played fun if both players are happy to move forward... Which is rare.
Parasoul... Napalm shot x n... Meh not as stupid as peacock imho.
Painwheel... Lets fly and see who can get the highest to launch an attack while above the other. And lets both up/back in flight all day... A ridiculous matchup if both painwheels are defense minded.
Squigly... Lets both zone with j.hp all day..... Ok this is fun...
Bigband... First to brass loses... Brass on brass action...sweep... Fun... No
Bella... Let's double jump around all day and try to get above each other... Yes yes
Double.. Battle of the j.hp and the j.mp to beat the j.hp. And lots of down back and dashin mixups. Not a terrible mirror just a bit slow if both players are footsies minded.
Fukua... Tennis with fireballs.. Well, more like dodgeball
Val... Possibly the best mirror when assists come into play... Just fundamental sg with lots of movement and pokes and zoning and counter zoning. Space for many different styles of play. As a strict mirror is probably a bit bad with both vals just double jumping all over the place trying to get a good look.
Filia... Ridiculous. Air dash spam all over the place, st.hk is tooooooo good cause it beats airdash and throw and cr.lk a dumbass mirror that all kinds of dumb things happen in... Gregor to max out the stupid.
Things like sweep to go under st.hk and then cancel into dp to punish st.hk become "viable" lol.
Fortune.. Pretty good mirror... And now that i think about it... Might be amongst the best in the game especially if one is a head user and one isnt... Though i guess technically that isnt a mirror. Head off mirror seems kinda dumb... But idk.
Marie? Yes... This has to be the dumbest mirror in the game nothing but skull on skull action... Revenge of the skulls.
Robo fortune... All the clanking of tin on tin action... Oh the sounds.. It will grate on the ears.
Beowulf... 2 dogs barking at each other and "wrestling" i guess it could be worse.
Eliza...a complete spacing rps game. First to block an iad attack loses.