Not trying to start a flame war here, buy what is the argument for using Rails over Express or Django/Flask for a new project ? Genuinely curious to know the benefits / downsides of each option
You can't compare Rails to Express. It's a much, much more full-featured web framework. JS doesn't really have an equivalent to Rails (that I know of). Django is comparable in scope but Rails is many times more popular and a lot more work has gone into it. IMO you would need a very good, specific reason to choose Django over the far more popular Rails.
Probably Meteor, though not as mature as Rails.