Blue blooms eventually fade so a rebloomer mophead hydranges may be better than one which blooms only on old wood. Best blue is a matter of personal choice. The shade of blue will also depend on how acidic your soil is and how much aluminum your soil has. Nantucket Blue might an easy one to try. It is a reblomer vesion of Nikko Blue (which only blooms on old wood and NB may have issues in cold zones).or the Together & Ever series. Check to make sure it is good in your zone though as these have trouble blooming in zones like 4 or colder.

Purples are mopheads too... that are very difficult to grow as they require near neutral soil pH or slightly acidic soil plus ideal quantities of soil minerals.

If you plant a mophead that blooms blue, the bloom is blue because of its potting soil. Planting it in your garden makes the roots enter in contact with soil that is different from the plant's potting soil. In time, the plant may then bloom a different shade of blue, purple or pink.

If your soil is acidic, you will get blue blooms. If your soil is alkaline, you will get pink blooms. To change pinks to blue, you will need to amend the soil with aluminum sulfate.

I have found Mathilda Gutges too be my best blue hydrangea....again depending on your soil

Where are you to the nearest large city? It matters as to what you can grow and how it will look as well as blue vs purple vs pink for many big leaf hydrangeas/H. macrophylla.

NH I don't understand what the proximity to a large city has to do with the color of the hydrangea. It is dependent upon the actual soil in your garden that determines the color. I have an Ayesha that has blue color and about 20 - 25 feet away a clone of the same shrub blooms pink. The pink one is right next to the cement pad that the heat exchanger sits. The pink one is also near the foundation of the house and the blue one is under the fir trees and gets a lot of the litter from the firs. The blue one gets a bit more morning and afternoon sun, and the pink one gets sun by noon and afternoon sun.
For the bluest in my PNW garden, I think it would be a tie between Nikko Blue and Heinrich Seidel at early full bloom, they both change as they age. Nikko Blue to light blue and Heinrich Seidel to purple/blue with chartreuse tones changing even as they are cut and in a vase.

Ophoenix, If jcmdmd had said Boston, s/he would have acid soil, for example, and if Tulsa, most likely alkaline, so often the city gives me some idea of soil pH in that area and for many H. macrophylla or H. serrata, that influences flower color. And knowing that you are PNW means you don't need to worry about hardiness, but here in NH the "best" is usually the one that is either hardy enough or small enough to grow in a pot and overwinter in the garage. Not much point in my trying to grow most of yours because I would never see flowers. Hence the location request. I don't know if jcmdmd is a beginner or someone experienced enough to know that not all hydrangeas are hardy in all zones and blue.