A13. Hub cities! I miss that more than anything else, that hub city was wonderful. Deadly at times, but serious fun. Barfing zombies. Huge hornets. You leveled up your crafting skills by ... are you ready? ... crafting. Want a better club? Go craft a dozen of them, and since you supposedly learn as you go, each club is a bit better.
A13 had a lot going for it, TFP could have just fixed the bugs and called it done for all I care.
A14 had seriously broken graphics, A15 was a nice bug-fix release. A16 didn't really have anything in it that pushed my buttons.
I am actually playing through A13 right now, reliving the hub cities and other fun things.