I had been using a regular gel pen for my Zentangles previously, and although it was alright for larger patterns, it didn't do too well for detailed work. It also didn't give smooth, even lines.
Recently a friend recommended micro tip pens and it has made all the difference. The different sized tips make it easy for detailed drawings and for filling in spaces. It is much more stable and even. They also come in coloured inks - I plan to get them once I enter Phase III.