I have read, most recently in Pamela Myers' excellent book Lie Spotting, that the words OK, Coke, and Shakespeare are the most universally recognized worlds in the world.
After Googling this, OK and Coke get the most hits, but I don't know if this is entirely accurate. Maybe in the English speaking world it is.
But in the non-English speaking world Coke is the only one with any worldwide recognition, even if it is known by its full name Coca-cola.
Having traveled to various Asian countries, including China, S. Korea, and Japan, I have noticed that "Bye-bye" is part of their everyday vernacular. OK isn't part of their lexicon.
I asked my Thai and Vietnamese friends, and they both informed me that both "Bye-bye" and Coca-cola are known.
Meanwhile, in my conversations with my Asian friends, Shakespeare is a known word, but most people don't exactly know much more than the fact that he was the West's most famous writer.
However, my best bet for most globally recognized words, and yes all three are English due to the fact it is the dominant language on this planet, would be: Coca-Cola, Bye-Bye, and Jesus Christ.
These words would most likely be followed closely by the word "Muhammad." Both religious figures make the list simply because there are billions of Christians and Muslims permeating the world, and are found in almost every developed culture. Additionally, Muhammad is the most chosen name to give a child in the world.
I would say the words CD, DVD, and Shakespeare are a close runners up.
If anyone has some actual data on this, feel free to post it in the comments section below. I would like to research this more. Also, I filter all posts with links, so don't freak out if the post doesn't show up immediately. It will take several hours before I can get around to checking the link to see if it isn't malicious and then approve it.