take a look at what Glenn Bartley uses for his images.
Some great images there. But he's mostly a bird photographer - I'm not sure how that equipment list holds up when you're shooting a wider variety of animals. Were his non-bird shots taken during dedicated wildlife shoots, or were they opportunistic shots taken of other animals while looking for birds, when the birding equipment he was using happened to suit the conditions? Also, I'm guessing he doesn't bring his entire lens collection along each time - he has a variety of lenses, but not every lens is brought on every shoot. But, if he were to have one big lens (and shot general wildlife, without a particular focus on birds) and had to rely on that lens as his primary lens (in combination with any number of small lenses), which would it be - the 600mm, 500mm or 200-400?
I've always found it a hard choice between the 400/2.8 and 600/4. You can only really bring one - each weighs almost 4kg (apart from the new, 'lightweight' versions from Canon and Sony), so any gaps in their capability will need to be plugged with small lenses. Outside of situations where extreme focal lengths are needed (birds, or shooting polar bears from ships and Zodiacs in Svalbard) I think it often comes down to the difference in AF and optical performance between a 400/2.8 with 2x TC at 800/5.6 a d a 600/ with 1.4x TC at 840/5.6 - and that's a model- and body-dependent thing that can't really be applied across the board to 400/2.8 and 600/4 lenses as a generic group.