My daughter's was "more", by which she meant she wanted more breastmilk. Next was "Daddy". It was a while before she said "mummy", but she signed it pretty early on.
I learnt signing with S but he never used it! Though once he was talking fairly fluently he would occasionally make the sign as he said the word, so he had obviously learnt it all just decided not to bother with the signs.... we never had any problems understanding each other even before he was speaking so I suppose he didn't see any need for the signs.
"More" is a great first word, very useful!