I shoot on a crop-frame Nikon (D5300) and I normally go to my Tamron 70-200 2.8 VC for conferences and events and outdoor portraits. I have also used the sigma 70-200 2.8 OS, although it has a minor compatability issue with my camera. The optics on the Tamron are reportedly equivalent to the Nikon counterpart, and the Sigma is not far off, and they are both about 800USD less. If you need it for shooting sports you probably do want to spend the extra and buy the Nikon version for the occasional times when the focus is more reliable and maybe slightly faster.
For indoor portraits I use the Nikon 50mm 1.8. For events, I would definitely say that vibration reduction (or optical stabilization / vibration control / image stabilisation…) is imperative for anything longer than 50mm, though I suppose it would be anything past 75mm on a full-frame camera.