I think Google Adwords is probably the best pay per click / online marketing program mainly because of the high market share that Google enjoys in the search engine market.

As Google has the maximum share of search engine traffic, it is clearly evident that Advertising in Google's online marketing programs will have the widest coverage and exposure.

On the other hand I believe that statistics show that other search engine like MSN and Yahoo have higher conversion rates. These statistics are however in terms of percentages so because of Google's share of the traffic, Google's programs may still receive a wider coverage.