I use Spark Plug brand earplugs. They have the best attenuation I have found. At work we have a variety of free earplugs, so I've tried a lot of different brands. To some extent the shape of one's ears are going to determine which plugs give the best results.
NRR ratings are based on a specific frequency distribution, biased towards higher frequencies which do more harm than lower frequencies. But if the noise is primarily lower frequencies (turbo-prop aircraft, loud V-twin motorcycles, etc), the NRR may not be valid.
I'm of the belief the more attenuation the better. Some sound is going to be transmitted through the bones in the skull/jaw, so it will never be silent no matter how perfect an earplug may be. And I never have trouble hearing sirens or traffic with earplugs in.