I am starting work with Jurkat cells and human PBMCs. After trawling through the literature there are three commonly used mitogens - PHA, PMA + ionomycin, ConA. Are they all as effective as each other? Or is there one better than the rest for cytokine production/proliferation?

It depends on what you want to do:
ConA is specific for T cells - dose dependent activation, clumping, cytokine production, proliferation - however, I found in one of my previous studies that excessive doses lead to apoptosis, while low doses result in a different costimulatory profile.
PMA+Ionomycin bypass surface interactions, work great on T cells, okay on B cells as measured by prolif/cytokine. Ab production by B cells is a bad readout for this mode of activation.
Ab based activation is usually more specific, but may not work as well for "bulk" cultures.