TLDR; 400 TC
I started out shooting wildlife with Canon, and then moved to Nikon. I've owned every single RF and Z telephoto lens that has been released. Some of the lenses I've owned multiple times as I goldilocks'd my way through. Admittedly I forgot this forum has shooters other than just Nikon... so my essay at the bottom is Nikon-centric.
if I were shooting Canon again, the choice is easy. RF 100-500 and RF 600 F4 and done. With Sony it is a bit more complex. I'd probably go 300 F2.8, 600 F4, and the 200-600. With Nikon, it's a whole different world with so many more attractive lenses.
I live in lower Michigan where we have few large mammals, so the majority of my local shooting is birds of prey (owls, eagles, herons, etc.) This is important, because somewhere like out west where you have bears, elk, moose, etc. will require a very different set of lenses. I do however go to the Tetons/Yellowstone at least once a year, and recently I've also been going to Hawaii/Iceland at least once a year. I will be going to Alaska in September 2025 to shoot the bears. So with a decent amount of travel - size and weight are important.
I'm 27 years old, 5'10" and about 205lbs - not in athletic shape. This matters, because I definitely prefer to handhold, and I prefer lightweight lenses.
The majority of my shooting is done near sunrise/sunset, where aperture is king. I would almost always prefer 600/800mm or longer, but I often have to make do with shorter focal lengths because even F2.8, 1/10, and 8000 ISO is quite normal.
Currently I have the Z 400TC, 600PF, and 800PF. I can't decide which zoom to pair with them, and also which PF lens to sell.
here is my quick summary of each Z lens:
Z 70-200 f2.8 - probably the most impressive zoom of the bunch. handles TC's great. generally a bit too short for my uses - really only valuable on destination trips (Puffins in Iceland, African Safari, Bears in Alaska, etc.)
Z 100-400 f4.5-f5.6 - probably the least impressive zoom of the bunch. has the worst performance of any lens I've tested with TC's. generally it is too short for my uses, especially given the TC situation. I really only value it when I am on trips involving whale watching - where even 100mm can be too long. it is a nice compact, lightweight lens, and if you are more into quasi-macro or landscape - I've heard high praise. good lens to have for travel or if weight is a significant concern.
Z 180-600 f5.6-f6.3 - for new wildlife photographers, I would call this a must have. the price to performance of this lens is outstanding. size/weight are the main downsides, but people have been using and loving the Sony 200-600 for almost 5 years now at similar size and weight. similar optical quality (in the center) as the 600/800PF. handles TC's decently well (better than many expected)
Z 400 f4.5 - another outstanding lens, at a great used price these days. often debated whether people should go for the 400 4.5 or 100-400. does the 2/3rd stop make a big enough difference to give up zoom functionality? this is a favorite walk around lens for me, as I find it to be a great 400 f4.5 or a pretty good 560 f6.3. a great option to replace an old 500 f5.6 PF. it's my 2nd in line to the 400TC for when walking local woods looking for owls in low light.
Z 600 f6.3 - this is the most fun telephoto lens I've used from any brand. if you have good light, this lens will not disappoint. to have 600mm at the size and weight of a 70-200 is incredible. it handles TC's very well, with the only downside being the f9/f13 apertures. it rivals IQ of the 400/600TC, and 800PF.
Z 800 f6.3 - another outstanding lens. the fact that it can rival the Sony 600GM F4 + 1.4x or Canon RF 600 F4 + 1.4x while being 2lbs lighter, 5" shorter, and almost $10,000 cheaper is insane value. biggest downside is the MFD. size/weight can also be an issue as it is larger than the 400TC. the undisputed small bird king. although I don't use it often, I can't seem to part with it. 800 f6.3, 1120 f9, and 1600 f13 are too versatile.
Z 400 TC f2.8 - I think this is the best lens in the line up. consistently the sharpest in tests and real world. produces amazing photos, and the flexibility of 400 f2.8/560 f4 is awesome. when I shot Canon, I never used my RF 400 f2.8 because it was often too short. the built in TC changes that completely for Nikon. It is like having both the RF 400 F2.8 and RF 600 F4 in one lens. when I see my own photos, and especially photos from better pros - this lens always stands out and can produce jaw dropping images.
Z 600 TC f4 - the jack of all trades, master of none. at MSRP, and with the 800 6.3 existing - it's a hard pick. the 800 6.3 is significantly cheaper and lighter, and offers almost indistinguishable IQ as the Z 600 + 1.4x engaged, with only 1/3 loss of light. I'd really only suggest this to people who mainly shoot at 600 f4, and value 840 f5.6 more than 400 f2.8. the 600 is the longest and heaviest lens offered in the Z line. the 11% increased weight and 15% increased length over the 400TC makes it that more difficult to pack, carry, and shoot handheld. I wanted so badly to love this lens (my main lens when I shot Canon was the 600 F4) but with all the other great Nikon options, I just don't see it winning in any value propositions. its main strength would be to replace both the 400TC + 800PF, if you can stand giving up 400 F2.8 and having the increased length/weight. I will probably own it again some time in the future, just to double check my thoughts because it was so underwhelming for me.
fortunately with Nikon and these offerings, there is really no way to go wrong. no matter what you find most important (value proposition, size, weight, IQ, versatility, etc.), Nikon has something that will be a great fit for you.