Antibody-drug conjugates (ADCs) make up a growing class of targeted therapeutics with important applications in cancer treatment. ADCs are highly modular in nature and thus can be engineered to target any cancer type, but their efficacy is strongly influenced by the specific choice of payload, antibody, and target cell. Considering the number of possible antibody-payload combinations, ADC development would benefit from an efficient method to narrow the number of ADC compositions to those with the highest and most universal potency prior to assessing pharmacokinetics and pharmacodynamics in animal models. To facilitate the identification of optimal ADC compositions, we describe the use of photoreactive antibody-binding domain-drug conjugates (known commercially as oYo-Link) to enable the site-specific labeling of off-the-shelf antibodies. This approach allows for the rapid generation of ADCs with a drug-to-antibody ratio of ∼2 with no subsequent purification required. As a demonstration of this approach, ADCs were generated with different combinations of tubulin-inhibitor drugs (DM1, DM4, VcMMAE, and VcMMAF) and anti-EGFR antibodies (cetuximab, panitumumab, anti-EGFR clone 425, and anti-EGFR clone 528) and were delivered to three EGFR-expressing cell lines (A431, A549, and MDA-MB-231). Real-time cytolysis assays indicated that the most effective antibody varied based on the choice of cell line: cetuximab was most potent against A431 cells, while 425 and 528 led to the greatest cytotoxicity against A549 and MDA-MB-231 cells. These results did not correlate with differences in measured anti-EGFR binding affinity as cetuximab had the highest affinity across all three cell lines, while 425 and 528 had the lowest affinities for all three cell lines. Panitumumab, which had the second-highest anti-EGFR affinity, exhibited the least effective cytolysis across A431, A549, and MDA-MB-231 cells. By demonstrating that ADC potency toward a given target is dependent on both the antibody and drug chosen, these findings can guide the selection of ADCs for further in vivo analysis.