When you look at robots throughout the history of pop culture, it's hard to ignore one of the biggest multimedia franchises of all time, and the droid that played an integral role in all six of the Star Wars films -- with more on the way.
R2-D2's lists of credits include assisting Luke Skywalker during the destruction of the original Death Star, serving as an invaluable asset to Obi-Wan and Anakin in the prequels, and rescuing the gang from Jabba the Hutt in Return of the Jedi. He's a mechanic, a communications device, a friend and pretty much whatever you need at any given moment; all that while communicating solely in beeps and whistles.
So why R2 over his long-time companion C-3PO? Well, though C-3PO is handy with languages and provides plenty of comic relief in the films, he isn't quite as ubiquitously useful as R2. Plus, R2's look is among the most indelible visuals in film. Remember the R2-themed mailboxes during the 30th anniversary of A New Hope? Who hasn't seen an R2 cookie jar, or the R2 cell phone introduced a few years back by Verizon Wireless? Go to a comic book convention and try to walk five paces without running into a remote-controlled R2 prowling the exhibit floor. Heck the droid even infiltrated the world of women's fashion, with an R2-D2 one-piece swimsuit.
In the world of robots, R2-D2 is its humble king.