I double up most of the time. But if I have to go with one only, it's the generic bright orange foam earplugs, the ones that feel almost moist or squishy. They seem to have a higher NRR than more dry foam or reusable rubber/silicone earplugs.
Roll them up to a thinner sliver, straighten ear canal by pulling the middle of the ear back to your rear while inserting. Of course, leave enough protruding to pinch to remove.
They're a little pricey but look up "noise fighters" on Amazon, gel replacement cups for earmuffs. They are like $40 each if I remember, and I don't remember what model ear muff they go with. But I hear they are very comfortable and transform ok ear muffs to great feeling ones.
By doubling up, it feels almost like I'm cheating, the noise is cut back so much I can really concentrate and build some good habits. I wish I had done it sooner, when I was first discovering my fundamentals. I can still speak and hear doubled up.