Cadenadeaventuras
I'd like to say Yin Chakra nature if it is included. I'd try to recreate technique systems from other media like Cursed techniques from Jujutsu Kaisen and/or Fear from Nurarihyon no Mago. Second pick would be Lightning. I'd persue development towards something like Chakra based Technomancy (kinda like The Rinnegan's Asura Path or Mecha Naruto for reference).
I would say Air and master its manipulations to the point of being able to control Atmospheric Pressure and learn now to control a vacuum. Just imagine it, you could boil people blood at room temps or Silently suffocate them by removing air from where they breath, and 100% nemesis to all fire as without air O2 fires cant happen. Maybe even control the gas mixtures in the air to only have Carbon Monoxide to poison people silently with no taste... But maybe my thoughts are a little to evil. Nah, how about forcing a small bubble of Air to allow internal reflection and make non-radioactive NUKES! NOW THERE'S YOUR EVIL!!!