The 'Exe version 1.1.2.6 build: 16:05:41 Jun 15 2009 (4554)' is the most used client in the history of Counter-Strike: 1.6
As you can see above is from 2009 and yet is doing his job fine without any bugs or crashes,
You can play Multiplayer (Internet and Lan) or with Bots,
Is compatible with Windows XP, Vista, 7, 8, 8.1, 10, 11.

The 'Exe version 1.1.2.7/Stdio (cstrike) Exe build: 19:53:27 Aug 3 2020 (8684)' is the latest update that we have from steam
You can play Multiplayer (Internet and Lan)
Is compatible with Windows XP, Vista, 7, 8, 8.1, 10, 11.