Which sign material should I choose: coroplast, PVC or foam core?

Coroplast (corrugated plastic): light, waterproof and cheap. It's the standard for yard signs, real estate, election and parking signs, usually with H-stakes. The flutes inside can show a slight line pattern in the print.
PVC (3mm): smooth, bright white, rigid plastic with a matte finish. It works indoors and outdoors and looks good up close, so it's a good choice for wall signs and longer-term displays.
Foam core: light and sharp-looking, but delicate and not waterproof. Best for indoor, short-term use like presentations on an easel or event signs. It can bend or warp if it isn't handled carefully.