City bike, touring bike, mountain bike... Basically every type of bicycle will do. The main thing is that your bicycle is in proper working order. As Holland is overall flat, the classical Dutch bicycle has no gears.
What gear do you need for a cycling holiday in Holland? Everybody has their own standard holiday packing list, so as far as your general travelling gear is concerned: bring what you feel is necessary. There are some essentials cyclists should remember to bring.