I wouldn't be without a pair of water-proof or spray-proof gloves, especially for handling anchor lines and things.
I've missed mine since smashing the buckle by running over it with the boat on its trolley
. Maybe not as useful on bigger RIBs with proper storage space...
The rest of my top five would probably overlap with others' lists: knife;
first aid kit; tools; PLB if I weren't always surrounded by dinghies and other safety boats; ropes of various sizes (for everything from improvised sunglasses straps and jury-rigging shackles to tow rope or anchor rope extensions).
And, of course, sandwiches and a flask of coffee!