If you control more than one creature with best in show, one of them becoming a target of a spell or ability you control (or a creature you control without best in show becoming such a target) will start a chain reaction. Say you control two creatures with best in show. The first becomes the target of a spell you control. That causes the best in show ability of the second creature to trigger. If you copy the spell, the second creature becomes the target of the copy, causing the best in show ability of the first creature to trigger. If you copy the spell, the first creature becomes the target of the copy causing the best in show ability of the second creature to trigger. And so on. Copying the spell or ability is optional, so the player may stop the award ceremony at their discretion.

If you control no creatures when Blue Ribbon's enters-the-battlefield ability resolves, the outside person won't be able to choose the most deserving one. Blue Ribbon will just stay unattached.