By staying on 1.6.640 with the corresponding SKSE link, you ensure:
SKSE64 Build 2.2.3 (for game version 1.6.640) Nexus Mods Mirror: Skyrim Script Extender (SKSE64)
Many modders intentionally "backport" or freeze their game at 1.6.640. This is because the transition from 1.6.640 to 1.6.1130 involved significant changes to the way the game handles plugins (the "Marketplace" update). skse 16640 link
While Bethesda has since updated the game to 1.6.1130 and 1.6.1170, the 1.6.640 build remains incredibly popular because of its stability and the massive library of compatible mods.
Grab the .7z file from the link above.
Are you planning to your game from the latest version to 1.6.640, or are you setting up a new mod list from scratch?
For game version 1.6.640 , you specifically need SKSE64 version 2.2.3 . Using a newer or older version of SKSE will cause the game to crash on startup. How to Install SKSE 1.6.640 (Step-by-Step) By staying on 1
If you see an error saying you are running game version 1.6.1170 but need 1.6.640, Steam likely auto-updated your game. You will either need to download the latest SKSE version (3.0.x+) or use the Skyrim Special Edition Downgrade Patcher on Nexus Mods to go back to 1.6.640.
SKSE requires Steam to be active in the background. Even though you aren't clicking "Play" in Steam, the app must be open. Essential Next Steps Grab the
From now on, do not launch Skyrim through Steam. You must run skse64_loader.exe to start the game with mod support. Why version 1.6.640 is a "Golden Build"