Very true...Entirely subjective. It's like asking what's the best shoe. The one that fits your budget, fits your needs, and fits your head is the "best" mask.
It also depends on what 'best' you are looking for. I don't believe there is one mask that is 'best' at everything. It really depends on how you are going to be playing, and what's more important to you.
Some masks have 'better' fields of vision. Some have better/easier lens changing. Some have better breathability. Some are more likely to get you bouncers. Some are lighter. Some are more comfortable etc...
Not a simple question to ask. In my opinion the 'best' thing to do is to try on masks and see how you feel about them.