I find that the best TextEditors can handle really wierd files.
I like a TextEditor that can load and edit binary files (particularly executables). I've done it with the following:
Favorite Torture Tests
My latest test is using a really long XML file (like an XML file with no newlines). Something on the other of 20 Megabytes on one line. Then tell the editor to put closing tags on their own lines.
Back in the old days, Greenview (makers of VEDIT) had TextEditorBenchmarks?