Metallic nanoparticle (MNP)-based solar cells (SCs) are considered promising candidates due to their enhanced optical absorption and substantial improvement in efficiency. One of the primary elements that significantly impacts the efficiency of SCs is the optical losses resulting from the reflection of incoming light incident on the surface of the SC. These optical losses can be reduced by using different MNPs, like Ti, Au, and Ag. In this work, we have introduced different shapes of these MNPs such as spherical, cylindrical, cubical, and inverted truncated cone (ITC), which are then placed over ultra-thin GaAs solar cells (SC). In order to achieve the best possible design requirements for MNPs-based GaAs SC, the finite element approach is used. Compared to the planar structure, the cubical structure of Ti MNP has demonstrated improved optical capabilities due to its inherent anti-reflection properties. The optimization of the geometrical parameters such as diameter, edge length, or base diameter of different shaped MNPs is attained in terms of the value of optical current density, Jopt. We found that the Ti MNP, having an edge length of 120 nm, has the highest Jopt and highest average absorption of 29.23 mA/cm2 and 92.33%, respectively, among various MNPs, such as Au, Ag, and Ti. The optimum Jopt achieved for the cubical Ti MNP is approximately 46.59% higher than that of the planar GaAs structure while exhibiting a reduction of 86.75% in reflection losses.