I like Fossify's, but I won't recommend it - it has no predictive text and no symbols on its main keyboard, the first which is a huge deal for almost everyone. Not sure if they're working on those or if it will never get those features, because if it had those two it would be the perfect keyboard because it's so great in every other sense.

the base aosp keyboard or florisboard

I just wish languagetool made an android keyboard, all the open source options require some compromise with the autocorrect in my native language