This question has recently gotten more complicated now that the USPTO has adopted the CPC, which is intended to be an international standard. However, the adoption thus far has been slow, and the large majority of patents remain classified only as IPC, ECLA, or US. My vote is currently for IPC or CPC (where applicable) since a concordance is available and both are international standards.

This question has recently gotten more complicated now that the USPTO has adopted the CPC, which is intended to be an international standard. However, the adoption thus far has been slow, and the large majority of patents remain classified only as IPC, ECLA, or US. My vote is currently for IPC or CPC (where applicable) since a concordance is available and both are international standards. The concordance is especially applicable at the class level, but will fall apart at the sub-class level, which relies more on ECLA subclasses.

The EPO and USPTO have switched entirely to classifying with CPC, and Google is planning on rolling out CPC classifications over non-patent literature within the next few months (source: PIUG 2015 CPC workshop).