Some of these challenges are downright peculiar.
Still, this is an easy one for me. Here’s how they rank, in ascending order:
Fire – meh, standard. Burns stuff. Good for trolls, but far too generic.
Lightning – Again, pretty, but meh.
Positive (Holy) – Very useful against undead, but otherwise it’s just shiny
Acidic – More interesting again, but not really my style
Necrotic/Shadow – Cool. Everyone loves a blade of shadows.
Sonic – Awesome fun. Destroying things with sound is great, but not very subtle.
I may have forgotten some energies, but I believe this only leaves me with:
Cold energy. For someone who wants a bit of bite to their blade without any showy, loud, or flashy effects. Useless against skeletons, but otherwise very effective. In terms of magic, frost is the controlling element, immobilizing, slowing, tripping and generally making people’s lives hell. I love it so much. Ice in general is my favourite element, and so while Sonic energy is hilarious, I prefer the more subtle, prettier, and more controlling energy type.