C.T.
Had plenty of experience looking for sippies that dont leak.
We've had best results with platex brand. The valves are easy to clean and replacements are easy to find if they wear out. I just passed on more than a dozen of these to a couple of friends. Some of them were 10 yrs old and all I had to do was put in a new valve.
I also had a couple of the Baby Einstein brand sippies that I found at the grocery store that had a silicone straw. They had a lip on the bottom to keep from tipping, the straw held up well and they closed without leaking in my bag.