I like Google Keep because it's one of the few apps I have found which gives me a readable preview of the notes. I generally try to keep the notes short. There is also an option to save the notes as Google Docs and then download as plain text if you like.
Of you wanted to arrange the short notes into a longer form, then Scrivener is a good next step. Again, Scrivener allows me to view these short notes in a readable preview. I can also drag and drop notes onto a larger document. Plus tons more features.
I have tried tons of other apps and none of them seem to allow a readable text preview. I like plain text, but I also like some sort of visual reference where I can scan loads of texts. Of course, this doesn't work well for longer text.
There have been some threads here lately about Perkeep, which I think has a lot of potential. A Google Keep interface on Perkeep would be perfect. What I like about Perkeep is that all content is content addressable and you can create meta-data external to the file which can hold things like tags, links between files, hierarchy, titles and a bunch more.