I think the best playground is my friends playground because it is very excitin.
So my friends playground very good.
Here's your corrected composition, Jimmy. Please take a look at the corrections below. See you later! 
Where do you think is the best playground in the world?Why?
I think the best playground is my friends playground because it is very excitin.
So my friends playground very good.
>>> I think my friend's playground is the best because it is very exciting. My friend's playground is really great!
>>> OR: My friend's playground is amazing! It's the most exciting one I've been to.