I have had 2 types on card, the first time I didn't put a title, it just had my name and contact details at the university.

The second time the department made them they put 'PhD student'. I got better treatment with the first type!!!

How about 'doctoral researcher'?

not that I am likely to get a business card.

I put 'Postgraduate Researcher' on the letters I sent out about my study, that worked quite well.

My view is that if you're getting a load printed, you might as well get something non-commital you can use after your PhD, like "biology research" or the name of your research group. I would worry slightly about putting 'student' on it if your PhD involved contact with companies etc.

I have name of research group and PhD candidate