I’d recommend getting Spidey and Thor as even though you have the previous unis, these two put out much more damage.
I had homecoming spidey at legendary and bought the IW uni when the update dropped and the difference was insane. Spideys main issue with homecoming is the low damage. That was completely fixed with IW uni.
With regards to Thor, one of my friends had Ragnarok at mythic with a lightning + 200 proc obelisk on him and I still easily outscored him in ABX every week with my poorly built IW Thor. I think I had a 120 proc or something like that.
However, if you don’t want to get these, Iron Man and X23 are both great unis, especially if you have X23 at T2. If not, get Widow instead.
Thor, iron man and widow are so frekin good with IW uniforms.
Killmonger is useless w/o the uni, mythic-ing that uni gives you a ctpE if you own black panther pack.
Cable has so much survivability and a dps boost with new uni which makes him S+ tier for PvE.
X-23 new uni makes her able to deal decent amount of dmg, and makes the character better overall.
I don't own Spidey and Hulkbuster IW uniforms.
You generally don't need another uni if you already have the best one for a hero. Thor is one of the exceptions since you can get a P-card with his Ragna uni even though his IW uni is by far his best one.