Cationic polymers are amongst the most utilized non-viral vectors for gene transfer owing to their ability to condense and protect the genetic material within polyplexes and to ferry it into cells. Quite a number of parameters, both related to the features of the vectors themselves (e.g. degree of branching, molecular weight, polydispersity) and to polyplexes (e.g. nitrogen to phosphate ratio (N/P), dose of complexes delivered, complexation buffer, etc.), are known to affect transfection behaviour. Consequently, some substantial discrepancy found in raw materials and in-home protocols across laboratories account for some disagreement and conflicting data about their performance. Hereinafter we provide a thorough chemical-physiscal and in vitro biochemical characterization, comparison, and optimization of the most widely used, commercially sourced polymers used in transfection, namely linear polyethylenimines (lPEIs), branched PEIs (bPEIs), linear poly-L-Lysines (lPLLs), and polyamidoamine dendrimers (dPAMAMs). By means of a stepwise approach, we pinpointed the most effective molecular weight and complexation conditions specific to each of them and correlated the physicochemical features of polyplexes with their transfection effectiveness. Besides, taking separately into account the effects on transfection of the plasmid dose delivered to cells, the cell seeding density and the volume of the culture medium, we highlited a range of optimal conditions roughly specific to each studied polymer. Finally, we coped with the effect of the variation of these three parameters at once on the transfection effectiveness of lPEI and bPEI and pinpointed an array of settings specifically optimized to attain truly superior performances.