I don't know if it was the best, but one of the codebases that really helped me out in the mid-90s was XEmacs. It was portable to many kinds of systems and had

I don't know if it was the best, but one of the codebases that really helped me out in the mid-90s was XEmacs. It was portable to many kinds of systems and had lots of features that most applications never have to worry about (e.g., an embedded interpreter). I also got value out of GNU Emacs but XEmacs tried harder to be a good citizen on the X desktop and there were lots of lessons learned in there that I was able to leverage for the applications I was building at the time.