Craigslist - You get cash, slightly less than you would get on eBay but you get your money instantly and a decent amount and don't have to deal with the sky high eBay/Paypal fees or scamming buyers. Don't be stupid about where you meet. Do inside a mall or major retail store.
Amazon - You get a decent amount as well but it's Amazon credit. Could be as good as cash if you're a big time Amazon shopper like myself. Another option is to sell it in their marketplace yourself. You just have to wait for the money to transfer to your bank account.
eBay - Selling it yourself on eBay nets you the highest amount possible even after fees due to the massive number of buyers on the site. You take the risk of the item being returned for 'not as described' or possible fraudulent chargebacks via Paypal. You can also use their 'instant sale' but I've heard many reports of them quoting people a high price and then giving a new much lower quote once they have actually received the item. They also make it difficult to get in touch with them if you want to decline the offer and get your phone back. Be weary.
Gazelle and other 'Instant Sale' Sites - I wouldn't sell mine here unless you're really afraid to sell anywhere else and don't care how much you get for your device. You can usually get almost double what they quote you if you just sold it elsewhere. Hell for some of the prices they're giving people '$250 for a 32GB 4S' you could sell your phone in 5 minutes with that low of a price on Craigslist.
Personally I would use Craigslist since it's the easiest and fastest way to sell it while still making a decent profit. Take good pictures and put your contact number (I use my Google Voice number instead of my real one) and you'll get a legit buyer pretty soon.