What constitutes a 'better' lens?

All sorts of ways to quantify "better". In no particular order nor an exhaustive list:

• Superior sharpness, contrast, flare resistance, CA, coma, distortion, vignetting, bokeh, etc.
• Faster and/or more quiet AF
• Wider aperture
• Smaller and/or lighter
• Less focus breathing
• Better build (e.g. more weather resistant)
• Internal Zoom

Of course, these "better" qualities may be at odds with each other. The most obvious example would be "smaller and/or lighter" and "wider aperture", such as a 70-200 / 4 vs a 70-200 / 2.8. The former is smaller and lighter whereas the latter is a stop faster. Many may well prefer the lighter weight and smaller size over the extra stop even if the prices for the two lenses were the same.

As for your third paragraph, well, that's outside my experience. I don't know what you mean by "only one [lens] that works for a given situation". That's like saying that there's only one car that works for a given situation. Sure, I wouldn't take an Audi A8 off-roading, but there are plenty of outstanding cars for performance driving and for off-roading. And while I may have a favorite car for any given situation, any number of cars will get the job done more than satisfactorily. Likewise, the same is true for cameras and lenses. More than that, it's easy to see that even if I did have a favorite car for a particular task or tasks, that another car might be made which I would like even better for the same tasks, which is true for cameras and lenses as well.

The bottom line for me is that, with the exception of very specific needs, any and all modern cameras and lenses (and many not-so-modern cameras and lenses) are well, well, well past the "good enough" point for the vast majority of people and situations. By "good enough", I mean that you did not miss the shot with what you had but would have gotten it with better equipment, and that the shot you got would not have been more likely to sell (or sell more copies), gotten more likes, or placed you higher in a photo competition. Now, of course that is simply not true for every situation -- there are definitely shots that better equipment will get you that less equipment will not, and there are definitely venues where better equipment will increase the chance/number of sales, get you more likes, and/or place you higher in a photo competition. But for the vast majority of people in the vast majority of circumstances, it is my opinion that it will not.

So, if my opinion (that modern equipment is well beyond the "good enough" point for the vast majority) can be taken as true, then what's the point of "better" equipment? To expand the range of your photography. With better equipment, you may attempt photos that you used to never attempt, you may print/display photos larger, you may take photos in a greater range of light, you may process your photos more aggressively, etc., etc., etc.. Or, you might not, in which case, "better" equipment serves no purpose for you.

its only been the last couple of years that high quality affordable lens have come to the market. eg: my tamron 28 75 2.8 G2 fits all the criteria that GB posted and more. silky smooth background blur that was better than my 85 1.8 sony prime. just bought the sigma 30 1.4 for my a6700 another cheap beautiful lens.

[edit] If I understand correctly ... if the picture from some crap lens is the "picture you wanted", then a lens like the fabled Otus 55mm is no better.

The trouble is that "a given situation" is infinitely variable - leading you to need an enormous number of lenses in order to posses that "only one" for any given situation.

Since you asked for an opinion, I take your post as railing against detailed discussion of lens characteristics or comparison thereof. [/edit]

A simple question that is difficult to answer.

You might consider all the technical properties of a lens like sharpness contrast, bokeh and other traits. But then there is build quality and mechanical properties like autofocus speed. Sometimes comparing two lenses is easy. My Nikon 24mm PC has infinitely better technical and mechanical properties than the Samyang 24mm shift lens, I once had the misfortune to own . It is better built too. But most of the time it is not that clear cut. Most modern lenses are surprisingly good, even budget grade models.

Is it better to take a long hike carrying a Panasonic 75-300 M43 lens or a Nikon 200-500? Maybe the reverse is true with a camera mounted on a tripod in a birding hide. Practical issues for our own usage may make a technically inferior lens, the better lens for us. I took one of my favourite pictures ever with the Panasonic 75-300 on a GX1, during a long walk. Until I bought the recent Z28-400, I did not have a long lens, light enough to carry on a long walk.

Later versions of a lens are not necessarily better, sometimes they are cost cutting exercises by the lens maker.

Of all the lenses I have owned, I have only had one lens I was not happy with. That was the Samyang 24 TS. Sharp enough, but I hated its low contrast. Others, including lenses panned by lens reviewers, have all done the job. Sloppy technique degrades the image far more than lab test lens quality. Even old bangers from the past like my Nikkor 35mm PC make nice pictures, if you take care of technique.

HAND.