In reply to kevin stephens:
For landscapes a ND for slowing down the exposure for that moving water look, you can go for a 10 stop for daylight to a 3 stop for dawn dusk
It's a nice effect but pretty overused at the moment.
A polariser is pretty useful, ones to fit a filter holder are pretty expensively, I use a 77mm screw on that fits both my most used landscape lenses which is a cheaper option.
Amazon is a pretty good source for cheap filters, it will allow you to experiment before you commit to a more expensive option