How to not buy the bad apple? I eat 2-3 apples a day and use them in various recipes - usually desert or pork related.  

I have found that I am literally terrible at picking out apples though and seem to be getting no better.

What I am looking for:


Crisp - this is a deal-breaker for me.  If it is mushy, I don't want it.  I have a hard time eating a slightly mangy apple no matter the actual taste of it.  These have to get mashed up into a dish.
Sour - I would like the apples to be as sour as possible - within reason.  I have never had an apple be too sour so I going with the assumption that anything goes here.
Sweet - Not overly sweet and not overpowering honey taste but sweet.
Thin skin - I like the skin.  I however don't like to be able to measure the thickness of the skin.


I am looking for what type of apples (I have a few favorites but even in those it is hit or miss), season, size, coloring, or anything else that helps identify a good from a 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!