What do you actually expect from this question? Obviously a lot of Rails users / enthusiasts are going to respond "Rails", a lot of Flask users / enthusiasts are going to respond "Flask", then I'm going to respond and say "Neither, use Grails", and so on. And at the end of it all, what will you know that you don't already know?
Whatever you're comfortable and already know. If you'd rather learn something new, then perhaps Elixir + Phoenix.
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