there are really no "best at finding whale sharks" on utila, every dive company sees them relatively equally. the skippers are the guys who find the whale sharks and they switch jobs from place to place relatively often, plus once a skipper does see a whale shark they radio everyone else and let them know where they are. so chances are, as long as your dive company goes to the north side every day (ask, before booking because some dive centers don't go to the north said everyday in order to save gas $$$) and if the weather is cooperating (must be dead calm to spot them, any white caps and its impossible), you will see them if they are feeding at the surface that week. the whale sharks follow the tuna (bonito) schools as they drive bait fish to the surface and the skippers look for the unusual movement of water at the surface (referred to as a boil or a roil) and the birds being attracted to it. you will then slip into the water and snorkel as the whale shark feeds on the bait fish just below the surface.
i've always had the best luck seeing them either the week before the full moon or the week after the full moon. although whale sharks are in utila waters all year round my best months for encounters have always been march, april, may, september and december/january. the key is to spend as many hours as possible out on the water to maximize your chances.