The choice of label for a secondary antibody is highly dependent on the application. 

An enzyme-labeled secondary antibody is the preferred choice for ELISA and immunoblotting. Horseradish peroxidase (HRP) is the most common enzyme used for labeling secondary antibodies. 

A fluorochrome-labeled secondary antibody is the preferred choice for assays such as immunofluorescence,immunohistochemistry (IHC) and flow cytometry, which require fluorescent detection. APC, PE, FITC, and PerCP are among the more common fluorescent molecules used for labeling secondary antibodies.