The GOAT of keyboards was Swype with Dragon Naturallyspeaking, which had shortcuts for adding punctuation so that you could type proper sentences without stopping your flow. It also had the ability to back up your personal dictionary online and a very easy tool for adding new words as well as swipe for cut-copy-paste. There was no competition for on-screen keyboards while Swype was functional and I would still use it if I could.
But Microsoft killed it and the APK doesn't install on newer devices. So fuck Microsoft with a knife for that in particular.
The only other keyboard I like is the Google Keyboard, which DOES NOT have online word backup but crucially among all Android keyboards DOES have a system that allows words to be imported. Most Android keyboards do not have this incredibly obvious functionality. For me to add words, I have a plain text file sitting in my Google Drive that I can add words to and then re-import. It's not the same and I will never forgive the loss of punctuation shortcuts, but since most keyboards don't have dictionary import, Google is kind of a winner by default.
I also demand Google Keyboard on every ithing as a prerequisite to me touching it. Apple's on-screen keyboard is an a tasty cheesy bacon and ranch dippin' abortion n' placenta sandwich of being bad at what it's supposed to do, mostly because it puts common punctuation keys on shift-access only.