If your load dissipates dc power as well as ac power (or rf power); then the maximum efficiency (and this is ideal) is 25%. This is the case for resistor-biased / dc-coupled transistors.
If the load only dissipates ac power; and you bias the amplifier so there are no resistive elements on the bias path; then the maximum efficiency is 50% (again ideal). This is the case for inductive-biased (through inductors, transformers, transmission lines) / ac-coupled transistors.
This is mostly mute; as you never really get close to any of these number. Heat loss, mismatch, device non-linearities (e.g. Vce to Vds ≠ 0) takes care of that.