As some of you may now know, my wife had our baby here in Costa Rica. The process was very smooth and my wife was comfortable through the entire process. Currently, we go to monthly check-ups here in Playas del Coco. Our pregnancy check-ups covered everything from listening to the heartbeat to looking at the baby on the ultrasound. Now, our baby is healthy and enjoying life every day. I tell you this back story simply to ease any thoughts of "maybe health care is sub-par in Costa Rica".
There really isn't one single best time to be in Costa Rica, all of the seasons in Guanacaste offer something special. Different seasons, and thus different weather, offer many different positive experiences from non-stop sunshine to tropical green rolling mountains. I would say that it really depends on what kind of lifestyle you're looking for. That will help determine if you would prefer to live here part-time, or on a more full-time basis.