I know this is an 'old' topic, but just had to say that I loved my Avent bottles - easy to clean and sterilise. I did have a bit of a problem at first, but discovered the reason was because I was over-tightening the rings. I actually started to use the avent bottles with MAM teats, and that worked really well, no matter how tight the ring! (I introduced bottles at 6mths as BF from birth, and lo didnt like the avent teats, mam teats are 'flattened').
I think the reason they leak if you over tighten is because of the anti colic valve. it distorts it or something.
Try using them with the mam teats, or not over tightening. HTH xx