Biro. Have got 530 and 425 written there at the moment.
I used to work with someone who really hated me doing it, she reckoned the ink was poisonous.
I've always done it, and I remember lots of people saying the same thing. I have those nice Uniball fineliners on my desk, but they are rubbish for it, wearing off in no time - cheap biros are best. I have the word "BILL" on my left hand, a shadow of which has just about survived the night's/morning's ablutions. And no, I do not have "BEN" written on my right, before anyone asks...