I like the traditional European flags - especially of those countries wich still have a royal flag. For example - the republic of Serbia still has its old monarchic flag, wich radiates dignity, respect and longevity. The twin-headed byzantine eagle of the roman emperors, the christian cross modifed with the serbian slogan, the royal crown. It's pure awsome. Few countries kept the old design. Croatia has a similar one. And Mexico is one example out of Europe, with a still traditional flag. Spain has also a nice flag.
As for simple but effective design and presentation, i like the modern flad of Germany, red, black and gold, with the Bundesadler to complete it. It's pretty neat. Coincidentally, the flag of the Kingdom of Prussia, Germany's forerunner, is my favorite state flag.