That question depends on where you are traveling and when ones experience is from. I switched from VZW to a T-Mo MVNO about 8 years ago (with a short stint on an AT&T MVNO in between) and had decidedly more dead areas. That lasted until 5G came out, and now I have fewer dead areas than I had even on VZW. Most notably campgrounds, which are notorious bad signal areas.
What I can't say, because I no longer have VZW, is if they have improved in the interim years as well. Or has AT&T? With the seemingly limitless variations on tower locations, antenna strength, topography, device specific antennas, the software driving those devices, it can be difficult to impossible to say what's "best."
If I were to do it again, I try the same thing did before. Cheap unlocked phones and prepaid service on throwaway numbers. When I was satisfied enough to change, I ported my real number to the new service.