Of all the choices[1], I’ve always been partial to the ISO 9 / GOST approach with Czech/Slovak-style diacritics (either version), even though those same diacritics make it impractical in most ASCII contexts. Just now I went to check the table and learned it actually prescribes ë for ‹ё›. Huh.

[1] https://en.wikipedia.org/wiki/Romanization_of_Russian#Transl...