Upload & Sell: Off
My vote is for the 500/f4, a lens I own; and which is my most used lens.
There is one caveat however. I also own a Sigma 120-300/2.8 zoom, and had it before I got the 500. The Sigma zoom is very similar to the Canon in terms of weight and bulk, just a little lighter and smaller. When I opened the box BH sent the Sigma in my first thought was "this thing is huge". After about a month I went to my personal trainer, showed her how I was holding the camera/lens and she set me up with some specific exercises that over time probably improved my hand holding by a couple of stops.
But I also know folks who can not hand hold a 300/2.8, they have to use a tripod. On the other hand there are folks who can hand hold the 600/f4 or even the 800; especially if they have a monopod, which to some extent I often use for resting when I am carrying the lens on a hike. There is a real learning curve in using the super teles as well, both with and without a tripod. I have modified my shooting style when using the 500/f4, and I suspect most guys will say it is true for them as well.
The bottom line is that your shooting style and physical condition are big variables in selecting the best lens. For a lot of folks the 500/f4 is the biggest lens they can man handle, and lug around on a hike. For others the 300 is all they can handle while for some the 600 is within their personal capabilities.