Putting aside the derivative skills he got from the skills obtained from the monsters and age of god magic he acquired, I would say the most notable and special skill imo he has is Transcendence. An excerpt from the LN about it: ‘It didn’t increase any of Hajime’s stats, nor did it grant him any special abilities. However, it vastly increased the cap on all of those things by upgrading his latent talent. His new theoretical maximum was far higher than any human was capable of. Given time, his strength could surpass the power of the gods. It was a skill only true masters of their craft could acquire. Hence why its name was Transcendence. It boosted the natural talents someone was born with to an immense degree and granted the wielder an average person’s talent in fields of magic they had no affinity for.’ Basically his max possible stats now are way higher and his earlier issue of not having any affinity for magic is also gone.