The Billboard European Hot 100 is by miles the most trust-worthy Euro compilation chart...
It builds up data from 16 European countries (the 16 most relevant countries to the music market), and it balances airplay, digital sales and physical sales....
But it's not airplay and sales from all countries, some countries Billboard only uses their sales, and some countries Billboard uses both airplay and sales. There's no formula whatsoever to balance the chart, for example a Top 5 in the UK practically puts you in the Top 10 of the Euro Hot 100 even if it's the only European country the song is charting...
but California Gurls is #1