There are SO many hot springs to choose from in this area and they all have their advantages and dis-advantages. Here's my take on each of the top 5:
1) Tabacon is the oldest and most natural of all the hot springs in this area but the price of not only the entrance but the drinks are just so high!
2) Hotel Los Lagos has some great hot springs & a cool pool, a couple of slides and several smaller pools people can spread out amongst them. Their bar/rest by the pool is okay and as with most hot springs, overpriced.
3) If you like to go where the locals go and don't need a bar/restaurant, go across the street from Los Lagos to Los Laureles. It's only $9 to get in (could be a little higher right now for Xmas) but take your own cooler with drinks, food (there's BBQs there so if you want to cook-bring charcoal & lighter fluid), and they've doubled the # of their pools recently, there are also covered Ranchos with lock boxes and power at each of them. The only key to going here is DO NOT GO ON THE WEEKENDS as it gets way too busy!
4) EcoTermales: romantic, very small area for the pools, great local food.
5) Baldi: If you want to party with a ton of other people and listen to loud music...this is the place you want to go...if not....pick somewhere else.
Hope this helps...enjoy this lovely area!!!