The top 10 data mining algorithms identified by the IEEE International Conference on Data Mining (ICDM) in December 2006, which are presented in this article and this subsequent book are the following:

• C4.5,
• k-Means,
• SVM or support vector machines;
• Apriori,
• EM or Expectation-Maximization,
• PageRank,
• AdaBoost or adaptive boosting,
• kNN or k nearest neighbors,
• Naive Bayes,
• CART or classification an regression trees.

See here for further discussions and viewpoints on the importance of more recent algorithms in data mining.