How to not buy the bad apple?
When I buy apples I also like to go for the most crispy ones. 
To find these, what I do is I flick the apples with my finger and listen to the sound it makes. You will be looking for a clear, 'high pitch' sound. Try flicking different apples and you will notice the difference between the sounds. The ones that are more mushy make a soft, 'low pitch' sound.
Hope this helps!