I think Osa is really the best place to see wildlife. As to how to get there and where to stay, I suggest you do some reading online or in a guide book to get some idea of what might suit you better.
You can start here: www.govisitcostarica.com
Manuel Antonio is not "the best" place to see wildlife by a long shot, I agree with mississippis post above that the Osa Peninsula would fit that bill, but it is one of the most user friendly places to see wildlife.
A delicate balance of rainforest and development is found in the area. I think its a good choice for a first trip and one many people seem to do. Arenal Volcano and Manuel Antonio are like the unofficial first timers trip stops, and it makes for a nice trip. follow Hatties advice about reading a bit more and the pieces will fall into place for you.
"Best place to see wildlife?" That depends on how far off the grid, away from nice restaurants and paved roads and swimming pools you're willing to go.
If the answer is 'not very' then Manuel Antonio is probably the best place to see wildlife. You are guaranteed to see sloths and monkeys in the park--seriously, guaranteed.
But, in terms of diversity and especially bird life, rainforests have more to offer, but they're harder to see there. Carara national park, just up the coast from Manuel Antonio, has harder to see monkeys and sloths, but you can see much better birds (especially scarlet macaws) and also crocodiles. The Osa has been mentioned, and other areas worth mentioning are the Sarapiqui region (the La Selva biological station is just off a highway, but you can see AMAZING wildlife there) and Tortuguero. We also had great wildlife spotting at the Arenal Observatory Lodge--the Arenal area itself isn't known for wildlife, but you can see a ton of it at the AOL.
Agree with MississippiGma...and if somebody knows about Osa is her...thank you and another vote for OSA...
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