My favorite cards are always changing in Marvel Snap. Here are my current favorites and why.
Sunspot is a great card if your deck leaves the possibility that you won't be playing a lot of cards and know you'll be leaving energy on the board. Helps to stop you from feeling like you're wasting energy.
Colossus is a great low-cost card that can win lanes that are problematic. Think about cards losing power, or being destroyed, none of which is a problem for Colossus. It's currently winning me Rickety Bridge about 90 percent of the time.
Spider-Man shuts down lanes. It's a great way to prevent your opponent from coming back somewhere that you're winning, or playing any cards at all if all cards must be played in a specific lane on a specific turn. If you can duplicate Spider-Man, you can shut down the entire game.
Currently my favorite card. I'm using a combination of Electro, Infinaut, and then TaskMaster. I can win the entire game with three cards, but having Sunspot on the board to soak up that extra energy is also cool.
Doctor Doom doesn't put a lot of points in one lane, but it can help you win lanes that are locked down. Playing five points in all three lanes on the final turn will win you more games than you think.