Contrary to what many players might think when asked about the best rated game on Steam, The Witcher 3 does not take the #1 spot. It belongs to Portal 2, which has around 98 per cent positive reviews, one per cent above The Witcher 3.
Valve don't release games often, but when they do, they make them the talk of the town for a century or two. Such was the case with Half Life, Counter Strike, Portal and Left 4 Dead. Sequels can prove to be underwhelming in general, but this is not true when it comes to Valve's sequels. For example, Portal 2 had about 88.464 positive reviews out of 88.228 total reviews on Steam at the time of writing, and Half Life 3 will likely topple that score.
Those two per cent negative reviews range from not-so-serious statements, over the game not being able to fulfil players' needs to people telling you to not look for negative reviews and buy the game. There are a few negative reviews that make sense though, referring to the game not working properly or not working entirely and they have all been posted recently.
The issues don't seem to affect everyone though, as most of the recent reviews still praise the game, with users visible being able to launch it in the last two weeks.
At the start of the article I compared Portal 2's reviews to those of The Witcher 3 which is widely regarded as the best game of all time. One fun fact is that The Witcher 3 is not even in second spot though, as that place belongs to Factorio. That said, The Witcher 3 is the only AAA game that made it into the top three and has about 60.000 more reviews than Portal 2 in total.
CD Project Red
Results on SteamDB and the actual Steam are somewhat different in both positive to negative review ratio and total number of review so there could some deviations from reality. The top five is rounded up by Counter Strike in fourth place and Portal in fifth. People really like GlaDOS, don't they?