I started playing around with KSP back when 1.2 released. To this day, 1.2.2 and 1.3.1 are my favourite "old" versions.
I spent much more time playing 1.2.2 than 1.3.1, since this was back when version downloads were separated into "stable" and "newest", and 1.2.2 was considered the latest stable version for a while. I eventually moved to 1.3.1, though, and I loved all the improvements it brought without changing too much. I was able to keep mods for a while, too.
I ended up dropping it when 1.4.5 released. Unlike with 1.2.2, I updated my game every time a major version received a patch (I usually skip out on the "X.0.0" versions), but none of them really felt as good as 1.2.2 or 1.3.1 did (maybe it was Take-Two).
I arguably think KSP 1.12 is a better game than 1.2.2 or 1.3.1 were, but I do miss back when things were a lot simpler and there were way less serious bugs in the base game.
I also don't own or care for the DLC very much, so that doesn't make much of a difference to me. But yeah, 1.2.2 and 1.3.1 were great.