Compact sizing of fin-and-tube heat exchangers is quite possible by integrating longitudinal vortex generators with the baseline fins. As geometry of the vortex generators is an important design parameter, the thrust of this three-dimensional numerical investigation is to identify the most favorable geometry of winglet type generators, if it exists. For that purpose, a thermo-hydraulic assessment of all the geometric designs, without any bias in the selection of potential contenders, is obligatory. Although past studies have reported the effect of winglet geometry on the performance augmentation, the selected designs were either biased or too less to draw conclusions. Each winglet design may be identified based on the geometric aspect ratio of the generators, which is varied over a wide range (0.5–1.5) in this study. Multiple favorable positions are considered, for installing the winglets around the tubes, before arriving at the favorable geometric design(s). Although highest thermal augmentation is delivered by the generators bearing smallest aspect ratio, linearized highest thermo-hydraulic augmentation is achieved when the ratio equals 0.85 irrespective of the generators’ position. After ascertaining the promising designs, their applicability is examined over a range of Reynolds number which spans from 2122 to 6367. Later part of the study discusses the change in the thermal and flow characteristics of the heat exchanger, due to variations in the winglet geometry. A study of the flow structures makes it evident that the spread of the longitudinal vortices widens, with eventual dissipation, in the flow direction, which has a discernible effect on the heat flux distribution over the fin surface. The study pertaining to the local changes suggests that the surfaces of both fins as well as the tubes undergo heat transfer augmentations, and the degree of augmentation grows with the decreasing aspect ratio of the vortex generators.