I know bending the rules is a bit of a unicyclist habit with these kinds of questions - about the one essential unicycle.
I do however feel that 36ers (and I guess 32” wheels) are in a class of their own and outside of the 24-29 realm.
This means that I do feel it would be better viewed as:
Which unicycle from 24”-29” would you have to ride, along side a 36”.
To my mind the following wheel+36” combo can never be bad choice/combo:
Combo A 24”+36”
Combo B 26”+36”
Combo C 27.5”+36”
Combo D 29”+36”
Why do I view it like this?
Well I think any of the above combos give the fullest experience of riding unicycles - and that comes from the fact that a Big Wheel ride is just completely different and separate to the other sizes.
I’m guilty of feeling I need a ton of different sizes. And there is a reason for owning each size. But it’s all about subtleties once you’re adding in wheel-sizes beyond one of the wheel+36” combos above.
I’d suggest the 36” is viewed as a massively different experience to all other wheels.
If I was new to this sport and was advising myself where to start I’d map out Combo D:
I’d say to myself:-
“First, Felix get a 29er and get really good at riding all sorts of trails. Towpaths, roads, muddy tracks etc. Then for your second wheel - because, there will be one more - get a 36er. Job done….”
Any wheel got after this is unnecessary but won’t feel like it is. That’s a different matter. The maximum unicycle experience I posit is achieved with any of the two wheel combo
*NOTE: I’m not including smaller wheels - trials and the like as I don’t use that size and would suggest they’re in a space of their own, much like 36er