I have fond memories of 0.16 as well, its also my first non-demo version of KSP I got. I remember getting my first 'orbit' in that version, which was extremely eccentric and still sort of dipped into the atmosphere.
My favorite version is 0.18, it added some pretty essential features like docking, unmanned probes, electricity, a more much better resource system, maneuver nodes and much more. Also, that version changed the brightly colored and cartoony aesthetic to a darker/low saturated 'realistic' aesthetic. I think 0.18 was the version that started the slow transformation into what KSP is today.