As others have said, it all depends on what you are after, the level of adventure sought, and your ability to speak Spanish.
In short, I would choose Guatemala. Amazing country! Beaches, cities, volcanoes, inland lakes, Mayan ruins, colonial towns. It truly has everything one could want from a Central American excursion. The downside is that it is relatively inefficient compared to Panama or Costa Rica and the tourist infrastructure - while it is there in select places - is not as developed as some may want.
If you want a 5* urban experience, visit Panama. It's interesting to see, kind of like Dubai in Central America, but apart from the skyline and the canal it is somewhat dull.
If you want an upmarket nature/beach visit, that's Costa Rica. Jungles, beaches and adventure-minded excursions are the name of the game in most of the country, save the capital. It's a relatively easy place to get around with only English and safety is pretty good, especially in the tourist zones.
If you want beaches and colonial towns with a more 'Central American' edge, go to Nicaragua. I really like Nicaragua, but keep in mind it is quite poor (second-poorest country in the western hemisphere). Despite this stat it is very safe, though infrastructure is, unsurprisingly, lacking. Unless on a package tour/with a guide, having some working knowledge of Spanish will help make Nicaragua enjoyable.
El Salvador and Honduras have pockets of interesting sights, but I can't recommend either country for a vacation. Just not enough to do and security in Honduras is abysmal (outside of Roatan).
In my mind, Belize is not in Central America so I will not comment.