what is a good guitar?  

You don't say if you play acoustic or electric hun. However, my partner has a wonderful 6 string Ovation acoustic guitar and also a 12 string Martin acoustic, both of which are excellent quality guitars. For electric, my partner has a Fender Telecaster, a Fender Stratocaster and a Gibson Less Paul, all top class instruments

I think the main point either acoustic or electric is that it has a good quality to be correctly in tune as for my experience mostly cheap guitars can´t be tuned perfect. As for an example I had a cheap copy of a Fender Strat and it never was exactly in tune. What I mean is that a note played on the 12 fret sounds exactly an octave higher as the note on the open string. You can work on the bridge saddles to achieve this but as I said on this cheap guitar I had screwed around for longer and still it sounded out of tune on most frets more or less. I guess that the frets where not correctly built in on the neck so I got my self a better guitar a real Fender Strat and the difference in quality was like between night & day.

On acoustic guitars I would also say that it´s importent to have a guitar that feels comfortable to play that means specialy the string height should not be to high. So on cheap acoustic guitars I have seen the neck curve wasn´t set right so the strings around fret 12 where far to high so playing in the middle & upper neck area wouldn´t feel comfortable at all. Some acoustic guitars have like the e-guitars a truss rod nut to set the neck curvature. For example if you press down a string on the first fret and the last fret at the same time then in the middle the neck should be slightly concave to the string. So there are acoustic guitars that haven´t got this truss rod and if the neck is either too concave, straight or convex then you have a problem or let´s say I wouldn´t buy this guitar.

ps. I hope this is not too technical, best is if you have someone who knows all this to help you as I´m german this is not to easy for me to explain in english. good luck

the Gibson SG is amazing.

Over the past 42 years, I have owned several guitars. For me the best guitars are:
1)tonally accurate up and down the fretboard.
2)the neck feels comfortable for my grip and fingers.
3)the guitar body fits comfortably with my body. Being physically on the smaller size, I am more comfortable with an auditorium guitar like the Samick OM7 or the Martin 000-16GT,rather than the larger dreadnaught or jumbo styles.
4)the sound produced is clear and clean.
Today,you can buy a "good" guitar for $300.00 but if you want a "great" guitar you can easily spend five times that amount! If you have the "ear" to hear the difference,it is definitely worth the investment. The two guitars I mentioned earlier are prime examples of guitars that I both find acceptable,but the Martin is tonally exceptional!
For electric guitars: the neck; quality of the pick-ups and controls are important. I have been very happy with the Fender Stratocaster and Charvel by Jackson.
In summary,the guitar you choose should match up with your personal needs and playing preferences. If at all possible buy the best guitar you can afford....in the long run it's worth it!

Gibson Les paul. Its so classy and has amazing sounds. But i think any guitars is good, it all depend on the player. Let the player make the guitar being better or worse.