1.14, the latest "official" version.
I sometimes try out snapshots but i dont really play much on them.
I also sometimes go back to 1.2.5 for some "nostalgia" feeling as thats the version i started on. Its also the last version that was ""single player"" as 1.3 version your worlds are always connected on "LAN".
I also played on 1.0 for some time way back then, but i found my self having more fun playing on 1.2.5.