Just about any of the IW uniforms are worth picking up, especially for Thor, WS, CA, BW, IM, Spidey, WM, SW, Thanos, Ebony, Corvus, SL, DS.
If you want to pick up some premium cards through legendary battle, consider the Ragnarok uniforms for Thor and Loki. Hela's and Hulk’s Ragna uni will each get you a CTP (and Hulk's uni makes him a valuable combat hero).
Other unis worth considering: QS, Venom (Anti-Venom), Shuri, Medusa, Emma Frost, Magneto, Cable, Vulture, Inferno. I would normally include Sharon Rogers but she has a new uni coming in the next update.
There’s more decent options but I’d recommend going with the heroes you enjoy most from those possibilities.
WS is a definite! It's top tier with his uni. War Machine is a DPS monster but he'll struggle to stay alive. Spidey is more geared toward PvP with his CC, Thor is another one who becomes top tier with his IW uni especially as you continue to get him toward T3. Black Panther? Skip. No worth the crystals considering for the 3300 crystals you can get damn near all the unis above with the 50% uni sale that's live right now