Rectification has been at the foundation of molecular electronics. Most single-molecule diodes realized experimentally so far are based on asymmetries in the coupling with the electrodes or using the donor-acceptor principle. In general, however, their rectification ratios are usually small (<10). Here, we propose a single-molecule diode based on an orbital resonance while using the highest occupied molecular orbital (HOMO) and HOMO-1 as transport channels. Our proposed diode design is based on an asymmetric two-site model and analyzed with DFT + NEGF calculations. We find high rectification ratios, even in the case of symmetric coupling to the electrodes. In addition, we show that diode parameters such as the operating voltage and rectification ratio can be tuned by chemical design.