Q : What are the most efficient solar panels available as of now?;

A: SunPower panels are about 22% efficient. Solaria, Panasonic, LG, REC are about 20-21% efficient and there are many companies (mostly cheaper brands made in China) that are in the 15-19% range. However, most efficient electrically is not the same as most efficient economically. For this, calculate the cost per kilowatt-hour generated by the array over the period for which the electricity output from the panels is guaranteed by the manufacturer. Today this is almost always 25 years. So if you have a 10kW array generating 10,000kWh per year (both fairly typical in MA) and your array cost $30,000 before the subsides and $15,000 after the federal and state subsidies then your cost of electricity (sometimes called the levelized cost of electricity or LCOE) is $15,000 / (10,000 x 25) or about 6c per kWh. To me this is the better way to look at efficiency and not the stated electrical efficiency of the panels.