If you could only keep one single IEM/headphone of your whole collection, which one would you keep, and why?
Personally, from the phones I own at the moment (SM3, RE-252, IE8, Radiopaq Jazz, HD-555) I think I would keep the SM3, because they offer the best balance between isolation, sound quality and comfort. I like my IE8, but only enjoy them during bedtime, low volume listening. They don't get much more eartime than this. Moreover, isolation is poor so it's not very good for commute.
The HD555 are simply outclassed by all my other phones (except the Radiopaq I bought as a backup). And finally, I really love the RE-252 on my Musiland DAC, and I think I like them even more than my SM3 at home. But outside (on commute), I find isolation could be better and they somewhat lack bass in noisy environment. So I would pick the SM3 as a general purpose IEM.