Well, you've got several choices and it depends on what you're doing.
First of all, if ANY byte value is possible, you can't use a delimiter. You'll need to use fixed length records. That's the only way you can make sure your data isn't cutting short a field.
Does your file have to be editable by humans? If so, you need to find a delimiter character that can be typed at a keyboard. Figure out which character isn't needed in the text, and prohibit it. If you choose soemthing like a colon (see /etc/passwd for an example), you'll have to filter it out of the data and prohibit it during entry. Again, depends on how you're getting the data.
If it doesn't need to be editable by humans, then you can use a character that can't be easily typed at a keyboard. See the site
for your menu of characters. There are already four delimiter characters defined. Look for FS, GS, RS, and US. These are Field Separator, Group Separator, Record Separator, and Unit Separator. I'm not completely sure of what their intended purposes are, but you could probably use one of these. You could also use a character in the extended character set, since those probably won't be in your data either.
Hope this helps.