Jo6Pack and MidlandWalther beat me to it...
... and JohnKSA added a thing or two I'd have put in the "nice to have" category, but they are good things...
... but for me, a "good gun" has reliability, accuracy, durability (which might just include quality parts and good fit/finish), and good ergonomics for ME (comfortable grip, comfortable trigger reach, smooth trigger, easy-to-reach controls, fast-acquiring sights).
Good ergonomics for me may be horrible ergonomics for somebody with shorter fingers.
As autos go, the best fit for me is the CZ75B or S&W M&P. As revolvers go, S&W K-frames are hard to beat, once outfitted with after-market grips or a Tyler T-grip adaptor.
As I think about it, the guns that best suit me are also rather attractive in appearance; so, for that matter, is my truck.... hmmm.... JohnKSA may be onto something....