What's the best separator/delimiter character(s) for a plaintext db file?

Well, there are few separator characters in US-ASCII, hex 1c, 1d, 1e and 1f. The plain text shouldn't contain them.

1c  FS  ␜  ^\  File Separator
1d  GS  ␝  ^]  Group Separator
1e  RS  ␞  ^^  Record Separator
1f  US  ␟  ^_  Unit Separator

No matter which character you choose as your separator, you'll want to escape any instance of that character in your data.

Perhaps tilde(~), or go to a high-ASCII character.

Either way, if there's any chance that it could sneak into your data, you'd want to escape it before writing to your plaintext file.

I think the best way to join string with a three cherries '@@@'.