There is no "best" wireless speaker. There is simply the one that works best for you.
The reason is...everything in this list will cause audio problems...
Microwaves
Cell phones
ADT
Cordless phones
Baby monitors
Walls
Distance
So we can offer you X speaker, but those may not work for you, then you need to try Y and Z.
My suggestion...
Buy X, Y and Z and try out all three, keep the one that works...return the other two.