Is there anyway to let RubyMine choose first completion choice like Intellij Idea?
Autocompletion's behaviour in RubyMine has been changed since there were a lot of complains against it in previous versions (because it was choosing not always the desirable one option).
If you want to restore the old behaviour, type "Registry" in Search everywhere and look for ide.completion.lookup.element.preselect.depends.on.context and unselect it.
Note: To Search Everywhere double press "Shift" key. Don't mistake this for searching inside preferences window.