It depends on how far you want to go in terms of technical vs narrative with your website. The most unbiased information you can get and present are technical specifications. You can find basic specs from most common sources like Wikipedia and various books on tanks.

Unfortunately bare bones technical specs don't translate directly into performance. Armor thickness and slope will give a good general impression of how well the tank was protected, but the manufacturing techniques and quality control when making the armor would all make a difference in just how well the armor would hold up in combat. Likewise when discussing the guns and ammunition of the tank in terms of how much power was really behind each round. Getting a very specific picture on how effective each tank's gun was (with what ammunition) would come from penetration tables, which would be made by government or corporate agencies conducting trials. You also need to understand the concept of the design of the tank since a tank can be put in many kinds of situations that it may or may not have been suited.

For these more specific criteria you want to look at testing and evaluation documents from the various armies and manufacturing agencies. tankarchives.blogspot.ca is a blog, so not a primary source, but they do have articles derived directly from source material such as CAMD RF 38-11355-817, a Soviet report on mobility trials of lend-lease and captured German tanks: http://tankarchives.blogspot.ca/2013/09/mobility-tests-of-lend-lease-and.html. This doesn't mean the source in unbiased because humans ultimately wrote it, but it is a primary source of an evaluation.

Scribd has original documents from various sources, including reports out of Aberdeen Proving Grounds like this evaluation of the KV-1: https://www.scribd.com/document/241820087/Preliminary-Report-No-10-Russian-KV-1

The British, specifically the Fighting Vehicles Proving Establishment ("FPVE" if you search for it in the archives), conducted and recorded many of their own testing and many of these reports can be found in the UK National Archives, although many of these reports may be unavailable online and copies would need to be ordered. For example the "Armour Trial Reports: volume 4; AT 66-75" can be ordered here: http://discovery.nationalarchives.gov.uk/details/r/C11273024 The archives website is not exactly the most user friendly but there is a ton of source material hidden there.

Taking that kind of approach will lead you to a lot of source material from during or just after the war from various countries. The Germans made their own detailed evaluations but you will probably need to be able to read German to make any use out of them.