Some serious website are evaluating flare.
It's very difficult to objectively measure it though.
Some other specialize on sharpness or, rather, MTF, which is way easier, but still subject to serious controversy.
Most are using target measurement method (dxo, Imatest are the most widely spread tools) but this is pretty limiting, especially with wide angle, due to target size limitation: you're limited to a distance of around 50x the final length, which is not representative of normal use for a wide angle lens. Lensrentals is the only one I know to measure on optical bench at infinity with at least 10 samples.
As you may see in the link I attached above, they also try to subjectively evaluate other interesting topics, like field curvature (a key topic for short, high luminosity lenses) or bokeh quality...
Yes, lens rentals are the best source I have found too for practical advice, particularly since they test multiple samples which shows how big the variance is. I enjoy the strip downs too.
I used to follow these things more closely, I did try to find the tests done showing how (probably impossibly) accurate lens registers needed to be to achieve the potential of the best lenses which I was sure I bookmarked, probably 10+ years ago, but couldn't quickly find it.
I have mainly used Leica since I could afford one. My used M6 went on for years. The lenses are not as expensive to own as it seems since most don't depreciate much and some appreciate a lot, so as long as the initial investment isn't too crippling they are a good buy. The digital cameras are very expensive if you update to the new model each time...
I used Canon SLRs from their debut autofocus EOS (I got a 620) I was never happy with their wide angles but the teles are superb.
I downsized to micro4/3 a few years ago, I had stopped taking my 600mm f4 because of the size and weight whereas the Olympus equivalent (in magnification and speed if not shortness of DoF) goes everywhere with me.
I also have a Fuji X-Pro2 as an in-between.
Now I am retired I don't get around as much as I did so don't take as many photographs. 40 years ago I was the only person taking family pictures, nowadays everybody takes gazillions of pictures on their phones!
). They are some hard-to-define qualities and in many ways are unintentional defects because of the engineering limitations in the past (the best example is Dallmeyer super six). Nonetheless, I won't deny that there are some magical artistic qualities of those lenses which are often absent in modern perfectly-designed lenses. 
