The woods that contain the lamppost in The Lion, the Witch and the Wardrobe. They're just a conifer forest, but very evocative in the book.
The Hobbit's Mirkwood is definitely a contender— it may not be either The Old Forest or Fangorn, but it's got a character all its own.
The Children of Green Knowe— not so much a forest as a very singular and terrifying tree.
Sherwood forest has its own special presence (despite being otherwise simply a real forest).
The Hundred Acre Wood holds a certain magic! 
Does the wilderness around the Green Chapel count? (Sir Gawain & the Green Knight.)
Still thinking… I seem to recall a story where Central Park is itself a fantasy wood. That's just a vague notion; however I did find Diane Duane's So You Want to be a Wizard (1983) and a series called The Wizards of Central Park West. I have read neither, but apparently a magical Central Park are elements of both. (Kinda fits, too.)
might, or might not, have finished the last units of his Volcano Dwarf army!
