During my main period of fieldwork in the Chinese countryside (1992–96, 2004-06), I had trouble with ballpoint pens, which often got clogged by the relatively rough copy paper I printed my surveys on. I tried fountain pens but the Chinese ink was not waterproof, and field notes may come into contact with rain, tea, river water, or Heaven knows what else at any moment, so it's best to use a waterproof or even archival ink.
I eventually switched to an inexpensive model of Rapidograph, which I bought in the US during a short visit in 1994. The fineness of the point was very helpful for the work, although I had periodic problems with clogging. Later, I discovered the Staedtler Mars marker pens, which have replaceable cartridges. I used those for a number of years.
Late in my fieldworking life, I began using Sakura Pigma Micron pens. Like the Mars, Microns are basically very fine markers, rather than true technical pens. But they tend not to become blunted or to dry out quickly, and there is no problem about refilling them or replacing a cartridge. Their ink is archival quality, which means it is indelible and pH-neutral. As long as you keep a spare handy, your fieldwork can proceed smoothly and your notes should remain legible for a long time.
I now use a computer for almost all writing and note-taking tasks, but I still carry Micron pens with me everywhere. There are several inexpensive sources for them in New York and on line.