As the holiday buying season starts I typically make at least a few recommendations or point out cameras that are a cut above the others. This year, however, I'm going to take a different tack.
First off, if you've already got a mirrorless camera (or even a Canon or Nikon DSLR), you should probably stick with your brand. Familiarity of control is a very important aspect to taking photos at a high level. Photos are moments in time, and you capture those moments either (1) randomly; or (2) in control, by not struggling with settings. I'd argue that if you're capturing peak moments randomly you should learn your current gear better or just buy random cameras ;~).
We now have at least three clear UX approaches (dials, button+dial, overloaded controls), and within that we have variations. Fujifilm and Nikon approach the dials cameras differently, for example, while Canon and Sony tend to overload controls differently. Couple this with things like the orientation of Command dials (Canon vertical, everyone else mostly horizontal), button positioning, and a host of other control differences, and it's a pretty complete new learning experience you'll encounter when you move from brand to brand.
I believe that you don't want to be learning a new camera, you'd rather want to be able to use a new camera much as you used your previous one, only now with more performance in some respect, or perhaps with new features you can take advantage of.
One can argue the feature bit, I suppose, as at any given product level you'll find one camera company having a slightly different feature set than another, but frankly, virtually every full frame mirrorless camera these days has a pretty extensive set of features that's tough to outgrow. Even in the crop sensor cameras this tends to also be true once you get above the US$1000 price mark.
Overall, in terms of new cameras, we are now back in that game of leap frog where whichever company most recently introduced their model at any given price point appears to be "ahead," but this tends to give the "buy the latest" crowd a false sense of superiority. Whatever "lead" the latest camera has over others that tends to go away when the other frogs leap again, and now you'll have a false sense of inferiority ;~).
Pick a lane and stick to it. I believe Fujifilm, Canon, Nikon, Panasonic, and Sony have all demonstrated that they're still iterating and their frogs keep leaping over others, so whatever gain you think you get from the latest and greatest is mostly ephemeral. Which gets us back to my first point: if you also have to master a new camera to get that gain, by the time you can take advantage of it the frog you were riding has probably jumped again.
If you truly don't have a mirrorless brand to ride as you move into the buying season—for instance, you're abandoning all your DSLR gear to move on—I still wouldn't put much emphasis on camera features or performance. Instead, I'd be paying attention to lens sets. Here, we have some more interesting stories that might make a difference to what you purchase.
First, as an early mover, Sony has an advantage in terms of how many different lenses are available (though there are also now a lot of duplicate and overlapping focal lengths). A four year+ head start meant that Sony had at least three full cycles of new lenses moving to fruition before Canikon made their first move. Broadly speaking, from conception to announcement, lenses from the camera companies take about three years to brew. First year centers on getting the design right, second year focuses on what's necessary to move it to production (including beginning to pour and polish the appropriate glass), and the third year puts it into production (first as prototypes to test, eventually as inventory to manage day one sales). Most of the camera companies have six to eight new lenses entering this cycle a year, which results in between four and eight showing up three years later.
Thus, Sony's early entry had them brewing plenty of FE-mount lenses before Canikon even woke up and created RF and Z. Sony is now on second generation iterations of many of their lenses, as well, meaning that they've studied how to make their initial offerings better while Canon and Nikon were still creating their initial offerings. If you value certain focal length and aperture combinations, the story here is that they may already in the E/FE mounts, but not yet in the RF and Z mounts.
That said, if you look at how fast Nikon ramped the Z-mount lens offerings, they're creating new lenses on a faster pace than Sony was in their first few years. As I write this, Nikon has 35 full frame autofocus lenses available, and third parties account for another 26. Which brings up another important point: Canon is still not allowing third parties to make full frame RF lenses. If you don't like Canon's offerings, you probably should avoid RF.
Meanwhile, Panasonic and Leica have partnership with the L mount that means you have multiple companies iterating for those cameras, including the third partner in the alliance, Sigma. This brought the L-mount offerings up at an even faster rate than Nikon's Z, though with a lot of duplication. The story here is that, particularly in the 24mm to 200mm range, there's plenty of choice, but as you move outside of that, the choices dwindle.
Canon appears to be restricting anyone from making full frame RF lenses (but not crop sensor RF-S), which to me makes their story more narrow than the others, at least right now.
Thus, more so than camera, I'd argue that you should take a long look at the lens lineup for a brand if you're thinking about moving into mirrorless for the first time, or switching from an existing mount. While DSLR-to-mirrorless mount adapters for the most part work (e.g. Nikon's FTZ adapter), I'm not a big fan of them as the additional mount means you need to be more careful handling your camera, as well as the fact that mount tolerances can gang up in a way that would require you to understand AF Fine Tuning of lenses.
In terms of specific cameras at different levels, I see a lot of parity these days.
For example, at high prosumer point we have the Canon R5 Mark II, Nikon Z8, and Sony A7R Mark IV. I can point to something each camera does that the other doesn't, or some performance aspect where one is slightly better than the other, but for 95%+ of photography you'd do with them, I'm not sure any of that really makes a difference. It would make more of a difference in how well you understood and handled the camera (back to my first point). Ditto the Canon R6 Mark II, Nikon Z6 III, Panasonic S5 II, and Sony A7R Mark IV.
For crop sensor cameras, there's much more variation going on, and it's difficult to line models up across makers. But even there, I've now used pretty much all of them, and it really starts to come to down to capability-at-price. The Fujifilm X-H2s and OM Digital OM-1 Mark II are probably the best choices at the moment for all-around capability that includes action, but they're also the two highest priced crop sensor models.
Thus, I'm not going to call out specific products this holiday buying season unless someone offers a XMAS deal that is so good you can't ignore it. As of today, there aren't any.
