What defines a good tonearm

As with most 'purist' audio concepts like 'valves vs SS'....'belt-drive vs DD vs rim-drive'.....'horns vs panels vs dynamic speakers'.....there is no one 'correct' solution or answer.
Rather, it is in the execution of a particular solution whereby a particular design may excel over another?
I have unipivots ( or dual pivots) like the Hadcock GH228, Phantom II and Copperhead.
I also have double gimbal bearing arms like the DaVinci 12" Ref Grandezza, Fidelity Research FR-64s and FR-66s and Micro Seki MA-505s and I have a double knife-edge bearing arm....SAEC WE-308.
You would think that if there were 'differences' inherent in the design philosophies of these arms alone...... they would be audible in side by side comparisons?
Apart from the Phantom II not liking high-compliance MM cartridges and the Hadcock not liking high-energy cartridges like the Titani, I find the differences between the various arms to be those of execution and quality.
There are far greater differences between cartridges than those between differently-principled arms IMHO.

Hiho, are you beginning to see the religious nature of all of this? ;-)

So much misinformation here.

A properly designed unpivot arm does not rock due to stylus tracking simply because the design has the stylus aligned with the pivot point; there is no torsional leverage and the only degrees of freedom from the stylus are the planes in the vertical and horizontal, which is what ANY arm has to have. There is no torsional "rocking" due to stylus motion in the two planes of a stereo recording going and what little of very low frequency may be provided from record warps and non-centered hole are effectively damped and remian unheard.

This is a very intelligent, reasoned discussion. It seems we all agree on certain major points as regards the advantages and disadvantages of unipivot designs vs fixed bearing designs. And I like that Hiho mentioned the issue of azimuth adjustment (AA) when it takes place upstream from the headshell (cannot avoid also altering both VTA and the angle in space between cantilever and LP surface). But that would be the same for both basic types of pivoted tonearm. As far as I know, the only "modern" tonearm that permits AA at the headshell itself is the Reed, when ordered with the optional AA headshell. I hope you unipivot guys will agree also that it is the cartridge that needs to follow the groove and that Ralph is quite correct to say that if the tonearm per se were to respond to the groove undulations, there would be no music. What lies in between those two extremes is probably what actually happens with most unipivots. Still, I have to explain the ethereal quality of the Talea that I heard locally. It made me want one. Obviously, the Talea "works". In thinking about what I heard from it, I came to the tentative conclusion that in part I was responding to highly euphonic "imperfections" that I have heard before with unipivots in my own system. I think this is what Mike was trying to get at; there is a certain sense of freedom (still not a perfect word for it) associated with unipivots that is very beguiling. And that's perfectly OK in my book.

+++++ " And what's wrong with that? " ++++

nothing, evryone is free on his choices.

regards and enjoy the music,
Raul.