A bit on the sarcastic side but
Ken Rockwell does his homework and writes informative and (relatively) honest reviews on a wide variety of products.
DxOMark has all the info you could ever want on the technical stuff. Most if not all lens companies provide
spec sheets for their lenses that give you a full technical breakdown of the performance (as it has been designed and tested by them). Most websites that you can order lenses and stuff from have some kind of comparison tool so you can see prices side by side. In the end of the day no one will simply tell you what to buy (unless you ask very directed questions) you must at some point decide for your self. I would be hard pressed to say there is a single source but the above listed are where I go for info.
Now some opinions:
In today's modern world
most of the specs don't really matter (all that much) for the
AVERAGE buyer. The drive down in tech costs has allowed manufactures to offer gear that is well above consumer grade to consumers. Yes you can go out and buy a D810 and some very pricey lenses to shoot birds on the beach. But you can also go out and buy a D3300 and a 25 year old 300mm AI-s lens and get more than professional results. Companies simply cant afford to release bad products any more (at least not like they could in the 50's). Was a Leica M3 better than a FED-3, you bet, is a Canon Mark (what ever they are on these days) better than a D810, depends if
@Derrel is involved in the discussion but generally ehhh no.
You should be looking for lenses that fit the kind of shooting you want to do not lenses that have some arbitrary performance spec. You may get plenty of suggestions on medium if not low priced ones because there are lots of great options out there these days for super reasonable costs. Dont be scared off by inexpensive or used stuff either, there is well over 100 years of photography gear floating around out there that can be had for great prices. You can find countless stories around the internet of "pro photographer uses amateur gear and takes great shots!". This title is a bit mis-guided as it really should be "Pro's use gear that considered pro grade 3-5 years ago to capture great shots!".
Take a look at what
Ken Rockwell has to say about sharpness and you will understand a good breakdown of what these specs are there for.
Regards
Dave