A.H.
Definitely the Playtex, they are primarily what we use. But it's all about the valve - that's what causes the leaks. I hate the Nuby cups!! Ugh milk everywhere. The Playtex cups are great because they have the rubber valves that fit in the top of the cup in 2 places (the top of the cup will have a small hole and a larger one). The valve fits really snugly inside there, and doesn't seem to get loose over time, just fits in there after tons of repeated uses (years!). I have another cup we use, too, and it's a thermos-type cup. Can't remember the brand but I remember the package said "for kids who are too cool for sippy cups." Too cute. Anyway the rubber valve on that cup is interchangeable with the Playtex cups (it might even be Playtex too) so it doesn't leak either.
And you can pick up the Playtex cups at pretty much any grocery store, drug store, or Target. :)