Personally I prefer the playtex drop-in liner in the 'antique' plain plastic/brown 'natural' nipple from the middle of the last century
I will use the regular liners that I've picked up at garage sales and am using the clear silicone nipples that playtex now makes cause I can't fine the brown nipples any longer and mine have rotted. I too use the gerber but usually as the infant gets a little older.
I do not care for the dr brown and do not think they make a heck of a lot of difference in gasiness. IMO. No experience with avent to speak of.
I have right in my policies that ANY bottles, sippies and toys that enter my daycare shall be considered a donation to my program.
I enforce this rule and have collected the above items at least once from each family. I use my own. I know that they are clean, safe and work well for baby.