I wanted to mention a new open source project Xed-Editor.
Additionally, I think the (virtual) keyboard you use is just as important as the editor (assuming you aren't using a physical keyboard). I would recommend Unexpected Keyboard I have had a great experience with it.
I'm going to try Unexpected. The swipe for symbol reminds me of my favourite on-screen keyboard, Fitaly. Unfortunately, nobody ever made it available for modern smartphones.
Edit:
The main thing I see missing is the option to swipe for uppercase. That may be possible in settings, but I didn't see it in a quick glance.