BRB, Carnage, or Knull

Are you talking about  PVE, PVP or both? I will answer based on PVP

Kitty, Wanda and BRB is an effective team. Maybe not the fastest, but it does work and is annoying to attack it if it is defending in pvp. Needs a lot of stuns once BRB gets rolling with Kitty beefing up the protect tiles. 

Kitty, Wanda and C5rnage works and doesn't work. You don't ever get Kittys countdowns to deal damage because c5rnage almost always ensures the enemy team always have 2 special tiles on the board. So there's that. And after maybe turn 5 or so, that is usually when Kitty will start beefing up your teams special tiles. 

With Knull, he almost always gives you additional strike tiles every turn and bear in mind your team's Strike tiles increase the value of Knulls permahealth damaging power. And Knull himself has a large HP pool, which is a benefit.  

I personally have had the most success with Wanda in pairing her with Colossus or with D5adpool. Her pink helps generate AP and boosts your team's powers damage based on how many AP you have in the selected colour. So technically it doesn't give Kitty much of a boost as both her red and yellow power deal no straight damage.

BRB works very well with Polaris. It's almost a guaranteed win against enemy Kitty teams

He functions well with Apocalypse too when you eventually get him. He hits hard on black and red, so you have strong powers on every colour but pink with those two.