Foam plugs typically provide the best acoustic protection of the common options with a typical reduction of 24-26. They are also cheap and usually easy to find. Their only real drawback is they can't be donned and removed quickly as the plugs must expand once they are in your ears. This is a problem if you're listening to instructions and then have to shoot immediately. The other drawback to ear plugs is that they become uncomfortable to wear for long periods of time. I haven't tried them but I'm told that custom molded ear plugs offer better protection and can be worn for longer periods of time.
Plain ear muffs are probably the most common form of hearing protection. They are popular because they can be donned easily and provide an acceptable level of protection. The price range on these varies greatly depending on the overall quality of the unit and the sound reduction it offers. My first ear muffs, which I have had for over twelve years, were something cheap purchased at a local discount store. I've used them for years without any problems and they are very comfortable and work well. I can't remember the manufacturer but they are the Model 1000.
I recently came across another model of plain muffs through my employer. They are manufactured by Height Leight and were originally intended for working around loud equipment - like chain saws - for long periods of time. These muffs offer an incredible noise reduction - 31 dB. They are heavier than standard muffs but if you must have the quietest muffs possible, these may be just the ticket. I carry these in my patrol car to wear when dispatching wildlife and my forty-five sounds like a cap gun.
Plain Old Ear Plugs
Effective Ear Muffs
Ear Muffs