I used Cello Butterflow pen from last 3 years.
I really love this pen because -
Quality of this pen is Awesome.
Smooth Running Pen.
Fast with full Comfort.
Long Lasting.
Ink colour is perfect. ( not too dark blue nor too light, perfectly balanced colour)
I also used other pens also but Butterflow is my all time favorite, I used butterflow in my exams also.