My son had a lot of trouble sucking.  We went to the store at 11pm one night, out of necessity-just to buy one of every bottle that was had a nipple closest to his pacifier.  This is what worked best.  He was born with a short frenulum (tonge-tied) and this was the only nipple that was long enough, and fat enough on the tip for him to be able to get ahold of.  They were super easy to clean and I would reccomend to anyone.