I used Cello Butterflow pen from last 3 years.
I really love this pen because -
1. Quality of this pen is Awesome.
2. Smooth Running Pen.
3. Fast with full Comfort.
4. Long Lasting.
5. 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.