Any model can do all of those things well. As to "best..."
For example:
A large display makes working on spreadsheets and image editing easier.
A large iPad tends to be more unwieldy to hold while taking photos than a smaller model.
While it can be great to have a huge display while composing a photo, I prefer a more compact camera in my hands.
I know what I prefer, but what's best for YOU?
Sticking with cameras, both iPad Pro models have the same cameras, so you can choose the preferred display size without compromising your camera decision. iPad Pro has a 12 MB image sensor and a f 1.8 lens aperture - basically the same as the iPhone Xs and Xr.
iPad Air, iPad, and iPad Mini are more affordable, but the camera is less capable - 8 MB image sensor and an f 2.4 lens aperture in all cases.
None of the iPads have the dual-lens configuration of the iPhone Xs.
So, how important is photography compared to the other features you listed?
Then there's word processing. All iPads have more than enough power to do word processing (as well as emailing, texting, Internet/Facebook, and spreadsheets). The decision will come down to other considerations than raw power.
For me, I usually write with a web browser open alongside my word processing window. Maybe you need/would like to do that, maybe you don't. All iPads can do that, but again, a larger display makes that easier/more effective.
I'm also a touch typist who prefers a physical keyboard. If you prefer a physical keyboard, keep reading this paragraph. If not, ignore it. While you're not obligated to use Apple's Smart Keyboard as your physical keyboard, size of display/iPad does affect the size of the accompanying Smart Keyboard. You'd want to try them on for size before making a decision.
In the end, I think the "best" iPad is the one you're most likely to use. In my experience, that has more to do with size than with other considerations. Is it too big to want to carry around on a regular basis? Is the display too small for your needs? Is it easy to hold in your hands?
If there was only one "best," Apple could offer just one model.