None of them are really great, Im hoping cosmic in the future will be, as it stands.
"Touch primary" DEs (IE. Phone, tablet with no keyboard or detachable etc.) I think are the way to go.
Plasma mobile is king 100% nothing gets remotely close right now.
Phosh I found far too buggy, and the apps are far too limiting. Things like squeekboard for instance don't scale properly, I had issues running chromium browsers on it too.
Ubuntu touch uses lomiri which im sure is great, I havent had luck running lomiri on any "common generic PC" linux distros. I did try getting it running on arch but found too many issues.
swmo is nice in theory, but it's missing a lot of the ergonomics.
Plasma mobile is missing a lot. It has some not great design choices I find, however it by far has the best app ecosystem in terms of actual app quality, as well as actually working fine on tablets and phones alike.
For touch secondary experiences I find KDE and Gnome to be just about as you would expect, Both are fine and mostly navigatable on touch only stuff, I would say KDE is often a lot better in terms of responsiveness, Gnome I can find bugs out as @that_[email protected] said. That being said, In portait mode, KDE is down right terrible some times as many KDE desktop apps have zero support for portrait aspect ratios, and you are relying on scaling being "low" enough that the app can fit fine anyways.
as for some stuff you can look forwards to in the future, We are starting to see stuff like catacomb which is designed for phones. I also had some actually great luck with Niri but it's mostly just buggy, and you still need something to manually launch keyboards.
In terms of applications, I have absurdly high hopes for cosmic apps. Each one I have used this far has a "tiling first" design policy, which translates pretty much 1:1 with being flexible for a phone it turns out, While cosmic apps have really poor touch support, if you install them and pretend your mouse is a finger, you will find that each one is almost perfect in a phone form factor.
EDIT: I wish I could say linux touch was in a good spot but realistically it's not. I personally recommend just installing something like Bliss or using waydroid as the primary experience. I am very active in the bliss community because in large part tablets and touch support. Android is still far better then linux is in general, even if a little less flexible, you can have a fully foss install like what I have personally.