They all have different features, so it depends on what you want.
ProCamera and Camera Genius seem to be kinda similar. I don't know which one is best because I only have Camera Genius but they both seem to have the same features (zoom, guide lines, big shutter button...). Camera Genius is slightly cheaper and from my experience it works very well, I can't tell you if ProCamera is any better.
Best Camera is a completely different type of app: while ProCamera and Best Camera are both for shooting pics Best Camera is for editing them with its filters and sharing them. I like Best Camera a lot but, again, its main purpose is for editing and sharing pics.
I haven't used Snapture so I can't help you with that one.