Very customizable (I like having number row always visible, and having long press for symbols on each letter).
Has gif support built in.
My biggest pain point is:
It auto corrects words that are spelled correctly to other words. I type "our" and it changes to "out", I recently typed "purpose" and it changed it to "purple". Autocorrect is awesome and necessary, but I hate that it corrects words to other words and not just typos. I wish I could find a keyboard that has the things I like without this annoyance.
Thank you for this (repeated) question! I will try some of these and collate my experiences.
SwiftKey
Long-time fan, in spite of privacy concerns. My bar for comparing everything below.
FUTO
First install, looks promising.
Indeed very customisable. What I don't like is the (imho) far inferior swipe typing and the need to explicitly switch languages for the keyboard to use the appropriate dictionary. Also, I miss directional buttons for those single-character position adjustments (Futo only offers space-key swiping). Voice typing seems highlighted but I find it to be unbearably slow.
Verdict: will most likely uninstall again.
OpenBoard
Installation somehow defaulted to "English (Australia)", but no biggie.
Seems very customisable also, but lacks swipe typing (a deal beaker for me). Relies on the OS language (actually, keyboard) switcher and curiously lacks a shortcut to its settings (requiring the user to go so the rest through the Settings app (which, best-case, is a whopping 5 taps).
Verdict: privacy aside, cannot compete with SwiftKey for features and usability.
Florisboard
Strainghtforward installation. Seems extremely customisable. No swiping nor autocomplete but both festures are clearly promised for a future release.
Verdict: apart from features promised in the future, thus seems an excellent keyboard.
Heliboard
Straightforward installation. Language selection included a github redirect to manually download dictionary, which was semi nice.
Proper big-keyed numerical keyboard. Also extremely customisable. Space-key swiping even supports vertical movement.
Verdict: apart from lack of swipe typing, probably the best contender!
Graffiti
Included because I friggin' loved it back in the day. The (to my knowledge) only app offering graffiti input is badly broken and crashes immediately on modern Android versions. I remember it working quite well on earlier versions, but that was years ago.
Futo voice to text works nice and fast on my pixel 8 pro. Fractions of a second slower than google. Also that's with the slower English 74 library (more data point, slower). They have an even larger one but the default is the smaller and faster English-39 model