I would recommend a registrar that would explain to the customer why they would not want a .me domain for anything critical unless the person lives in Montenegro and trust the Government of Montenegro to maintain a good and trust worthy registry.
Otherwise just use which ever registrar is cheapest and who you think will handle any quirks or shenanigans that registries may do to domains you own, and which own system and processes hold high enough standard for you.
Otherwise just use which ever registrar is cheapest and who you think will handle any quirks or shenanigans that registries may do to domains you own, and which own system and processes hold high enough standard for you.