just like there is no best radiator, or best pump, or best cpu block.
There is no best reservoir. Because each has an application fitting its need.
The overall recomended tho is the MCRes from swiftech due to its size and function. its a great res at a cheap price. Some will say EK, however they all have horrible vortex issues.
I like my koolance res, however cleaning is a PITA since you cant remove the top.
So to each his own is the proper answer. YMMV!