On the North Pacific Coast, Playas del Coco will certainly the best choice for the period of Mai through to November since you have the widest choice of dive sites to chose from: Catalina Islands about 15 miles/20 km South, the local dive sites about 2 to 5 miles away and the Bat Islands about 30 miles/40 km North. This is also the period when you have the best visibility (10m/30ft to 25m/75ft) and the warmest (25C/80F to 30C/100F) and calmest (no or little wind and waves) water.
If you want my personal opinion, the Cast rarely have anything additional to offer than the local dive sites off Playas del Coco with the exception of a change when you dive several days. Besides this you might be looking for some more challenging conditions since it's possible to have currents, surge (caused by big, smooth swells) and maybe greater depth. Marine life is pretty much similar, when you're lucky you might see a big school of either Devil- or Golden-Rays, Mantas are occasionally very common to see (best from November through to April). The presence of any other sharks than White Tips have still to be proven (I have never seen any other, not even nurse sharks), and you can see plenty of these on the local dive sites, too.
The Bat Islands are probably, no, it's for sure, the best dive sites all along the Pacific side, nowhere else you can regularly see the Bull-Sharks during this period, but you also have Mantas and massive amounts of schooling fish, sometimes huge schools of eagle-rays and all the other common creatures... You can get there only form Playas del Coco and only form May through to November, it's best from June to September. But the conditions can be tricky and require good experience (more about this in the specific thread about the currents out there). Just that much, be honest with yourself, and even if you are a good diver, to start out with a local dive excursion is wise even if the conditions at the Islands are most likely similar to the ones of the local dive sites it's better to be well prepared with weighting and awareness of the conditions (viz/temp/ect.) for more enjoyment.
Hope you'll find the perfect spot, and after all, every dive is great!