Tough call. Sahaquiel, Iruel, and Arael were awesome, but I'm going to have to vote for Leliel. I just sort of have a thing for things that distort things, especially when those things distorted are space, perception, reality, and/or gravity. Liliel satisfies half of those.
But still, dropping yourself from orbit? Cool. A rapidly evolving, infectious, living circuit board? Awesome. Overloading your targets brain? Epic.
I went for Ramiel just because of the upgrade it got in 1.0. However, if we're going by how the angels appear in the series I would probably go for Leliel or Zeruel. Sahaquiel is amazing as well, especially in 2.0
Ramiel not only because It was made awesome in the rebuild but also he is based on a real angel, and as do all angels have a greater purpous in the anime from psychological aspect :) It has awoken a certain feeling of protection ...