BitPay is also in early discussions with three of the top 25 websites ranked by Alexa, which are testing BitPay's systems, he said.
3 out of top 25. This calls for wild speculations but as you said, pick any 3.
If I were to pick the best: amazon it is and we will never ever need any other argument when asked if bitcoins really exist.
The "worst" pick would be Wikipedia donations (finally) but (excluding the non-english sites) or some other technical corner application. Anyway I'm curious to see it actually come true. There is too many reasons for speculation and announced announcements in bitcoin for my taste.