Well lets consider what you'd like to learn with your first AD carry
-Proper Last Hitting
-How to Poke
-Comboing with different supports
For this reason I think Ashe would be the most effective. It's the ADC I learned on.(mainly because there we're only a few at that time) and I would still recommend her. I don't feel like playing corki/trist/ezreal are going to make you the best you can be. While those easy escapes will cause you less frustration but with one flash every few minutes I think you'll learn proper positioning more quickly
Poking is easy with volley and you're not going to get any easy kills early against less skilled players, which is a good thing in my opinion. Her natural slow will also give you alot of experience for kiting like you would on other adc's with a red buff.
Ashe also hits like a pea shooter early on without a crit, so she can help naturally train some patience. Learning on Ashe should make any adc feel like second nature really.