Sunpower says they are the best at around 21%, but there are new panels coming out now from Centrosolar in Arizona (German company) that are called CIGS-and they are supposed to be around 20%. Thing is, most panels are in the 14 to 17% range and they are generally much cheaper, so the efficiency isn’t everything.
Mono Crystalline solar panels are more efficient than poly Crystalline solar panels.
SunPower panels have the highest rated panels in the market. However, there are multiple measures of efficiency for each manufacturer, and it is reflected in the costs.