What defines a good tonearm

Stanwal,
Don't misunderstand me, I think the ingenuity and craftsmanship that goes into something like a triplanar or graham or schroeder or any of the other high end arms is well worth the money spent. Much like sophisticated medical or lab equipment, these low volume works of machine art are expensive to create and duplicate. This isnt stamping a shape out of a piece of tin, but real machineing of different metals pieced together to form a working product.
My question is more why is there two basic lines of thought on how to make a pivoted tonearm. Either unipivot or captured bearing and what is the inherent deficiency in each that the other is trying to fix.
My assumption is that a unipivot is more free to move in all directions, and is therefore more likely to track the groove accurately. I also assume that a captured bearing is less likely to allow a cartridge to chatter or bounce around outside where it is supposed to be. And then we have the Townshend silicone trough to damp all this motion, which does seem to work from my experience, although I havent tried it with a unipivot.
Is it your opinion that either of the two pivoting tonearm types is the right answer, and that the other type, while it can be made to sound great, has an inherent impediment to perfection, if that was achievable.

I think we had a similar thread which was deleted. Unfortunately, because it had everything, from the idea of an enthusiast with a knife in the left hand and a piece of wood in the right one up to engineered knowledge about geometry.
but whenever someone feels to make something, analog equipment is the future.
Personally I like those who did not sleep the last 5 years because they made research to serve the discriminated Audiophile. Unfortunately they don't do it for free. A pity.

All mechanical systems have deficiencies and designers take different paths to alleviate them. I disagree with your major premiss that one or the other arm designs MUST be superior to the other; the critical part of your statement is "if that were achievable". It is not! We do not live in Plato's ideal world but in one where the ideal does not exist. I have had , sold, set up , more arms than I can possibly remember and found that there are good arms of all major types. Worrying about which arm more closely approaches the IDEAL arm is much less fruitful than simply listening and deciding which one reproduces music in the way you think it should be reproduced.

Stanwal,
That would also be Plato's ideal that we could all just sit around and have all this equipment available to audition and make our own decision. We dont. So another option is just to keep buying $4000 tonearms until we find one we like, but we will never know until we try them all, and then something new will come along. This thread was an attempt to narrow that focus down.
I am perfectly happy with the SME 345 I use now, and my system gives me the best sound I have ever heard. But I have to assume that it isnt the best sound available or even the best available for the amount of my investment. But since I am happy with the rest of my system, I was looking to upgrade my tonearm/cartridge starting with the tonearm.
And my premise is based on no two things ever being equal, so one must, by definition, be better than the other. Now, what is "better" is another question, but certainly there must be certain design parameters that are attempted for all tonearms, and each design accomplishes those goals differently than the other, with each having tradeoffs. My question is simply, which one comes closest to the ideal, mechanical and physical goal and why?

I repeat, there is no "best" arm. Attempting to judge sound quality by design parameters is impossible; what you are apparently searching for is a way to objectify a subjective judgement. If you like your present arm keep it until you hear something better or know why you are dissatisfied with it. Again your major premiss is wrong; there are not "certain design parameters that are attempted by all tonearms"; there is violent disagreement about pivot design, arm mass, straight line tracking vs pivoted and a myriad of other elements. There is a voluminous literature on all this that you can read at your leisure. The statement that if two things are different from each other then one must by definition be better is meaningless; the meaning of "better" will depend on external factors not defined. Ordinarily a million dollars is "better" than 10 gallons of water but not if you are lost in the desert.