A lot of people swear by GoldEd, which surely must be the most fully featured editor on the Amiga.
Personally, I reckon you can't beat the original (before a different author took over) version of Blacks Editor (the updates became unstable :-( ).
Why? Because:
* Its super fast! It doesn't have a custom GUI and excessive bells and whistles. Its a no-nonsense editor that is perfect for programming.
* Everything is scriptable, and every menu option is an ARexx command that you can change!
* It just gets the job done!
I don't know where you'll find the original now, I think Aminet only has the newer versions that are unstable here. I would make an archive from my installation, but its a bit heavily customised :-(