Hi I have done CR three times and planning my 4th.
We did Tamarindo and Manuel antonio first
Arenal and osa second
and toured central valley and montezuma the last time
I liked arenal and it was a neat experience if you think you are going to do costa rica only once I would go there and then do a beach destination. Out of the beach destinations tamarindo I liked the least although it had many bars places to eat and tourist friendly town.
Manuel antonio area was beautiful and more lush than tamarindo although many hotels are not directly on the beach lots to do in the area and more animals than tamarindo
Osa is very special just what you think of a rain forest a little more out of the way and tons of nature and wildlife I loved it there. The town of puerto Jimenez is not very touristy and not convient to alot of the hotels
Montezuma I also liked great town lots to do and fun bars and places to eat the scenary is beautiful. The town has a hippy vibe too it which I loved.
What are you most interested in? animals, beaches, or adventure and go from there.
In arenal do not miss the hot springs they are great and zip line at sky treck if you go . Cano negro tour is also nice.
We always travel private driver or fly if you watch you can sometimes catch a sale on domestic flights I havent found the cost too bad for the convience.
Here is where we stayed in each location and I would recomend them all
adventure inn in san jose if you have to overnight by airport
capitain suizo in tamarindo or sueno del mar B and B just outside tamarindo
si como no in arenal or lost iguana in arenal
yellow coco lodge in osa or iguana lodge
I have not stayed at bosque del cabo but it is on my list check it out
montezuma beach house or ylang ylang in montezuma