Of the 3 flag extensions I tried, this is the best one if you're looking for just a visible flag, as the flag is the best rendered. It doesn't give as much information as myip.ms' "IP Whois & Flags Chrome & Websites Rating". If you could combine the visibility of this one and the info provided by myip.ms' extension, it'd be a great combo.
Shows country flag and other IP / domain information in the location bar
This extension shows a country flag for the current website's server
The tooltip shows city, region and country name
The popup shows more information about the domain
Details are Provider, Alexa and Domain ranking
The design of the popup is simple and clean
To protect your privacy, we only send the domain name to our database