In my opinion, the best card is 'refill energy to max' because with it there are so many ways to get unbeatable builds for example:
Cards
-Refill energy
-Next attack is bouncing
-Next attack is love
-Shoot 9 Bullets
Buffs
-Bouncing attacks are also piercing
Optional buffs:
-Attacks shoot 1 more bullet
-Regenerate hearts
-Love lasts 2x longer