Bellow is the best first pick due to Wyrm being the guaranteed second boss, but the best perk is actually Peer in general.
Below gives you more area, and in later stages it pushes enemies away and even damages them.
Drain drains health in an area, duh, becoming larger at later stages.
Peer does nothing at the first stage, but from stage 2+ it gives healing items, generating them faster at later stages which basically means you don't need to buy heal. Pair that with Bellow and you can survive forever
There is something you can do with all 3, just take a good look at where the eye is pointing, and where the chip on the red box is pointing. You'll figure it out pretty quickly from there I assume.