Mercedes-Benz was the biggest seller of luxury cars globally in the first quarter, monthly registration figures showed on Tuesday, putting it in the frame to knock BMW off the top spot this year for the first time in a decade. Mercedes, owned by Daimler, has not been the best-selling luxury car brand since 2004, but is surging ahead thanks to growth in China, the world's largest car market, where its sales jumped 36.4 percent in the first quarter.