The concept of single molecule rectifiers proposed in a theoretical work by Aviram and Ratner in 1974 was the starting point of the now vibrant field of molecular electronics. In the meantime, a built-in asymmetry in the conductance of molecular junctions has been reported at the experimental level. In this contribution, we present a theoretical comparison of three different types of unimolecular rectifiers: i) systems where the donor- and acceptor-part of the molecules are taken from charge-transfer salt components; ii) zwitterionic systems; and iii) Tour wires with nitro substituents. We conduct an analysis of the rectification mechanism in these three different types of asymmetric molecules on the basis of parameterized quantum-chemical models as well as with a full non-equilibrium Greens function / density functional theory (NEGF-DFT) treatment of the current/voltage characteristics of the respective metal/molecule/metal junctions. We put a particular emphasis on the prediction of rectification ratios (RR), which are crucial for the assessment of the technological usefulness of single molecule junctions as diodes. We also compare our results with values reported in the literature for other types of molecular rectification, where the essential asymmetry is not induced by the structure of the molecule alone but either by a difference in the electronic coupling of the molecule to the two electrodes or by attaching alkyl chains of different lengths to the central molecular moiety.