The Howler Monkeys are a favorite of mine. They are called howlers for good reason. Their deep jungle howl can be intimidating for sure.
We have tromped all over different parks in Costa Rica but always get a good look at them in The National Park of Cahuita. The little place we stayed was right next to the entrance. Pretty easy to get up early enough and not miss these guys swinging in the trees above us.
They put on an amazing show but boy do they move fast. We had to pretty much run to keep up with them as they swung above our heads. One morning we saw the National Geographic photographers there. I am sure they got a lot better images than we did.