I was going to recommend the nuk with the clear soft top. I think sippy cups take some getting used to. My DD is breast fed she has never really taken a bottle, other than a little bit of water a few times. We started giving her sippy cups at 4 or 5 months now at almost 8 months she is getting good at drinking from them.
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Mine seems to do the best with the ones with a silicone top. She drinks out of Dr. Brown's bottles, and I think that having the soft top is a good transition for her. I tried an 8 ounce sippy cup, and it was too heavy for her to hold. Handles also help. There is an Avent transition bottle that is 5 ounces, with handles and a silicone nipple. This seems to help her figure things out.
We also have the Nuby with the soft silicone spout. He mostly chews on it, but seems to be getting the idea. We also have a Playtex hard spout. He can kind of suck on that one, but he looks at me like he is confused most of the time. I just give it to him every few days so he gets used to them.
Re: sippy cups recommendation
DD was ebf and did great with tommee tippee (around 6 months old)
DS uses event bottles and had better results with a soft silicone nipple like NUK. (around 6 months old)
They both started using straw sippy cups around 7-8 months and preferred that over traditional sippy cups.