A suit (skirt or trouser) would be great (I wore a skirt suit to my PhD interview). I always err on the side of caution and would much rather be over- than under-dressed for formal occasions. I think dressing smartly shows that you are enthusiastic and serious about the interview (and are not taking anything for granted!). However, if you are not keen on wearing a suit, then a smart pair of trousers or a suitably formal skirt with a shirt or a (smart) t-shirt would also be fine. The most important thing is for you to feel comfortable and confident. Good luck!!
P.S. You might want to search the archives on here too, this question comes up a lot!