For me, it’s a two tie between Jaller (who I owned when I was a kid), or Hahli (who I also I currently own).
I also really like Toa Mahri Matoro (who I just got a few weeks ago) and I think Matoro’s Mahri form is a perfect representation of who Matoro is and what his personality is and I think it suits him more than his Inika form.
I’d also say Jaller, he has nice colors and the crab is neat.
While he definitely got the short end of the stick, I also really like Hewkii. The blaster arm is cool, his mask is very unique and keetorange + black is always a good combo.
Storywise, Matoro for obvious reasons.
Visual appeal, Hahli because it was awesome seeing a winged water angel front and center in all the promotion images.
Set wise, Nuparu, black and silver with a glowing green tank on his back, plus his shield. Very fun to swoosh around.
Jaller because crab bazooka