Do you mean most common, or most typical? I'm going to bet that, for a couple decades now, people use whatever headphones come with the device they bought to use with headphones, and not many headphone users buy headphones specifically any more.
I remember when the small foam-cushioned headphones that came with the original Sony Walkman ended up being a ubiquitous design. Millions of those were sold, either because Walkman-style cassette players became ubiquitous and each had a pair of headphones packed in, or because the fact that they were lightweight, cheap, and many of them sounded okay made them popular in the aftermarket.
If you mean the most common single item sold now, it's easily whichever version of earbuds are packaged with Apple's portables this year. Note that Apple's earbuds have been redesigned a few times since the first iPod.
These days, first place for aftermarket phones are probably low-end IEMs (little better than earbuds with nozzles to fit in the canal, really), since those can be had for cheap in every department store, grocery and convenience store.
