Just started playing KSP again, what version should I use for mods?

[Neil Kermstrong]

noticed the game has had quite a lot of updates since I last played, so many mods are going obsolete or not working, so I'd like to know what is a good version to use to install mods. 

[Spengler]

I'll start off with "YMMV (Your mileage may vary)", primarily because it's a matter of personal preference for what you are looking for both out of the base game AND what mods you want to install and use successfully.  Me, personally?  I'm running 1.8.1, and I have no issue with the base game or the mods.  The mods I've got installed and are working most awesomely are:

• AtmosphereAutopilot
• Kerbal Alarm Clock
• MechJeb 2
• Module Manager
• Reentry Particle Effect Renewed
• Transfer Window Planner

I'd say the most important thing is to identify what base game features you want, and then go from there.

[KSP_Recreations]

You should probably go to version 1.11. I'm running 1.11 and RSS works perfectly fine.

 

[DeadJohn]

I last played KSP with 1.11 and didn't have any big conflicts with ~80 mods loaded.

I have yet to test 1.12 with mods. Some of my favorite mods are in the middle of big refreshes to add new parts, so I'm waiting for the authors to finish that, and I suspect they'll deal with 1.12 along the way.