By "good" I presume that you mean consistently reliable, reasonably accurate, and sufficiently powerful for personal defense use.
By "cheapest" I presume that you mean inexpensive when compared to most others available.
Based upon those two presumptions I would recommend the Makarov (Russian, Bulgarian, or East German, military rather than commercial models), the Polish P64, and the CZ82. All are reasonably available at attractive prices and are chambered for the 9X18 Makarov round, something like a .380ACP on steroids. Simple, durable, reliable and effective for the purpose.
You could also look around and probably find a very good used S&W Model 10, 12, or 64 in .38 Special for under $300, in which case you would have a very good handgun for personal defense, and ammunition would be both easier to find and less expensive.