Nikon 70-300 VR Nikon 80-400 VR Sigma 50-500 OS Nikon 300 f4 (older non-afs) with and w/o TCs
Without comparing lab tests, here's my real world assessment:
Both the Nikon 70-300 and 80-400 are plenty sharp. I prefer the 70-300 because, for the situations I want it (travel, walk-around--carrying around for long stretches of the day) it's lighter and I usually don't miss the extra 100mm in those situations. That said, the sometimes maligned 80-400 is an excellent lens as long as you understand and can work around the slower AF issue. As mentioned, it's not the choice for tracking BIF or for action sports. You can use a 1.4x TC (not the 2x) on the 80-400. You don't want to use any TC on the 70-300.
The 50-500 is in a different category in terms of size and weight. You can still handhold it for shots but you don't want to do it a lot. I use it on a monopod. Sharpness is comparable to the other zooms. I currently use it for my wildlife/birding work.
Both versions of the NIkon 300 f4 (af-s and non) give you the best sharpness for critical work or pixel peeping. They play very well with both 1.4 x and 2x TCs. No VR so a tripod is a must. This was my birding set-up until I went with the compromise and convenience of the 50-500 focal range.
With good technique and skilled pp, all these lenses can provide outstanding results. It's more a matter of what you will be shooting and your own preferences for size, weight and handling.