There are a number of ways of measuring how well a PV module performs, one of the most popular is to look at the cell or module efficiency of the panel.
Two of the best performing panels in the this regard are those made by Sunpower and Panasonic (formerly Sanyo), who currently both make modules with the highest of efficiencies available on the open market. Most crystalline silicon PV modules operate at efficiencies of around 15%.
The Sunpower E20 327 PV module is
the most efficient PV module on the market with panel efficiencies of just over
20% however, there is a cost associated with the improved efficiency and the
Sunpower modules are one of the more expensive on the market. Fortunately BenQ also make a PV module very similar in design and efficiency to the Sunpower. The BenQ Sunforte range of PV modules also has 20% efficiency allowing more power to be gained from smaller roofs.
The Panasonic (formerly Sanyo) HIT series contain a slightly different technology, HIT stands for Heterojunction with Intrinsic Thin film. Instead of just single silicon cells the HIT series contain a "sandwich" of two silicon cells with a layer of amorphous silicon in between. This produces modules with efficiencies of 19% in the case of the VBHN240SE10 module.
Both these modules make claims of improved performance over other manufacturers in terms of energy output at high temperatures, however this very much dependant on local weather conditions.