Turntable versus tonearm versus cartridge: which is MOST important?

@pindac , with the concentricity spec on records at +- 2 mm I would not worry about turntable runout. I think as long as rumble is low and speed accuracy is high you are in business. I certainly agree that per precision is always welcome and the way all three items interface is very important. The toughest part is isolating it from the rest of the world. 

I'm from the camp, "Garbage in Garbage out" So many people I know that listen to my system say, " wow! those speakers are great! " As far as I'm concerned although a well balanced chain contribute to the end result you hear, It's the cartridge, tonearm, turntable or cd transport DAC that contribute the most. .Another analogy, No matter how "good' the camera, film is, if you start with a ill focused lens,,,,,,,,then what will you get as a photo?

As in all things HiFi, much of what is being created by one within their system, is based on how previous experiences have been a influence.

How would one know an improvement has been made/encountered unless they have a recollection of something that has been perceived as being bettered.

As I am mainly a vinyl user, and have a investment around it, that is for my means quite a proportion of a disposable income, the extra cost associated with working with a Bearing is nominal and the impact a improved Bearing has had is quite something to me. I can't at this present time see how the incorporating of a improved design for a bearing has anything but beneficial and valuable, I know without reservation there is a better environment produced for the other owned supporting ancillaries to function in. My system is where it is and I don't foresee much is to change in relation to devices, my monies today are more available for improving on the mechanical interfaces that enable the best to be attained from a vinyl replay. 

The measurement for a Platter run off is a improvement as a result of a Bearing Design being improved, but the real real benefits, the one I attach to the noticeable improvement being perceived, is the selection of materials used to produce the design. The materials selected allows for new and tighter tolerances for machining over what was previously in use. The machining tolerances and lubricant used are  resulting in a reduction in noise being produced during the bearings function. As said the improvement has the impact to the point it being present feels tangible.

Adding further materials with known properties to manage in a improved manner, what energies are present, takes the overall impact to a further level, but not as the Improved design for a Platter Bearing will have.

Each to their own on the subject of optimisation of mechanical interfaces.

From a commercial view, the tasks needing to be put in place to achieve the condition is time consuming, requires a specialist resource skill, and is costly, resulting in the method being quite expensive to buy into. As not many/if any Commercial Brands are willing to share their tolerances and measurement parameters, not many know what they own in relation to tightest of machined tolerances and remaining with extreme low friction during operation. It is a case of willing oneself to believe the owned Brand has incorporated machining and materials that are the best for the role selected that is available. 

I’ve heard examples of both extremes.
The ultimate responsibility of the tonearm depends on the characteristics of the cartridge.