We have tried SO many different bottles (we're in Canada, and have an avid spitter-upper, gas-guzzler, and reflux baby).
So far, here's what we've found:
Dr Browns let her gulp in a lot of air ( I think this is due to the shape of the nipple - she tends to latch more loosely around the long nipples), BUT, she burps quickly afterwards. The pain is that we have the 3.5 oz size, and all my formula measurements are for 4 oz - a pain! I have also read that they tested the plastic Dr browns bottles that were supposed to be BPA free, and they found them to contain BPA!
Nuk - the flow was way too fast for her - even the slow flow - she chokes a lot every time, but is just starting to take this in the afternoons when she's hungry and alert. They compare to the Avent IMO, but don't leak as much. I take these to go.
Avent - the flow is great, but man, if you don't have it screwed on JUST right it leaks - had 4 oz of h2o leak down my back (diaper bag is backpack) and then had no sterile water for LO's bottle. I use these at home for expressed BM
Nuby silicone - love the look of this bottle - and that it does what the ventaire does without having as many parts, but the nipple is still too fast (it came with a medium flow nipple)
Playtex drop ins - found these difficult to get an accurate measure of formula in, and since we sterilize the water, fill the bottles then add formula as needed and shake, I always worried that the ratio was wrong. Also, my LO only has small amounts of formula for now, and we have the bigger drop ins, and the liner doesn't collapse - I think the size has to correspond more closely to how much you are giving.
Lastly, Playtex Ventaire - a PAIN to clean, as there are so many little parts, but this is the one that my LO latches onto the best (we use wide). The nipples are truly slow flow, so she gulps less, and if we use these at night and for dream feeds, we don't even need to burp her, so she doesn't wake up. Also, I like the 6oz size - gives us room to grow - not too small.
Hope this helps!