After using quite a few different style/brands of slings, I settled on the MagPul MS3 Single QD Sling. I prefer it because it's easily adjustable for length while wearing it, and depending on what your doing, can quickly convert from a 2 Point to a Single Point sling in seconds without removing it. I also have a few Vicker's Slings from Blue Force Gear that I like as well, but just prefer the MS3.
MS3(R) Single QD Sling GEN2