It all depends. If the spinning reel (also known as open-face reel) that you have is a newer one and the cost is more then $40.00, I would suggest braided line. If the spinning reel is very old and costs less then $40.00, I would suggest monofilament.
The reasons why: Braided line itself usually costs more then $20.00 to put on a spinning reel and definitely not worth it if the reel is under $40.00 or not made to use braided line, which very old reels were not. Monofilament usually costs under $5.00 to put on a spinning reel. There are pros and cons to both braided line and monofilament. But the best thing to do or I would advise you to do, is go to one of the local tackle shops and bring your spinning reel with you. They will examine the reel to determine the condition it's in and tell you exactly which line will be the most effective on your reel. Just make sure you tell them The Online Fisherman sent you!
If for some reason you don't want to do that and your spinning reel is brand new, just send me the make and model of your fishing reel and I will gladly advise you what the correct line should be put on your spinning reel. However if it is not a new spinning reel I will not be able to evaluate the condition of your spinning reel. This is a very important step in determining what line or if line should be put on at all. So the tackle shop is the way to go.
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Captain David M Rieumont
You can check out Captain David himself from his incredible and very popular Patriotic Angler podcasts. They cover every bit of fishing you can imagine, and there are new ones being created constantly. David and his team remain one of best resources any fishing community has anywhere in the country.