Which are the most efficient solar cells?

Soitec and Fraunhofer Institute has produced PV cells with a 46% efficiency. These are called "concentrator quadruple-junction" cells. They are very expensive and are used in space applications where power is critical.

Sharp produces a type called triple-junction solar cells with an efficiency of 44.4%.

A Spanish solar research institute (IES) produces a two-junction cell with 32.6% efficiency.

SunPower creates modules with a 21.5% efficiency.

Q-Cells hold the record for thin-film solar with a 17.4% efficiency.

First Solar creates thin-film cells with 17% efficiency, but they are the Cadmium Telluride (CdTe) type of thin-film cells.

There are even more categories and subcategories but this page (in Links below) has a great graph showing efficiencies over time for different categories and subcategories of PV cells.

Links

1. Solarlove.org.
2. Wikipedia has a good page on many types of solar cell technologies. It also has a table of the price of PV cells in US Dollars per watt. The US is the most expensive right now. US prices of electricity run from 9 to 12 cents per kilowatthour. Keep in mind that in some countries solar power is heavily paid for by the gov't, i.e. taxpayers. This brings down the cost of solar power for the consumer.
3. Wikipedia also explains solar cell efficiency.  
4. NREL is the US National Renewable Energy Laboratory. It has a section on solar power, and is the source of the graph above. This page has an an updated graph direct from NREL. This links to the actual image. They also have solar maps which show you which areas get the most sunlight. More solar maps here.