My two best spots are the Rio Colorado (Atlantic side of Costa Rica) which I prefer in August,
September and October, and yes it can be rainy then but the bite is always good.
Canas Negra which borders the Rio San Juan between Costa Rica and Nicaragua and is close enough
to Lake Arenal to stop over for a day of Guapote
(Rainbow bass - most likely a relative of the Peacock Bass of Venezuelan fame) fishing.
I make this trip twice a year, December and June.