We use the Take and Toss straw cups (sold at Target and Walmart). They can be a pain...when DS remembers he can take the straw out it drives me crazy but I like them overall. We try to just use them for smoothies because milk and water just get too messy. These are easy to clean and easy to get together. I also find that a lot of straw sippy cups make it difficult to get the last bit of liquid and these straws go right down to the bottom allowing them to get it all out.
I have just been using a plastic cup with a screw top lid, the kind with the straw that has a ring at the bottom so it can't come out. It's the kind that you see in all the grocery stores that is made for adults. They are kind of heavy, but Kate thinks she's hot stuff drinking from a big people cup, and the lid and straw can't come off/out, and the straw is wide enough for smoothies. I found one one at Target that has a swirly straw and is a little smaller, so it's easier for her to hold.
Re: Best Sippy w/Straw for smoothies!