I use a variable nd circular filter by marumi. It has 8 stops as you rotate it, ie it darkens as you rotate.
I have found that it works very well at 2, 4 and around 6 but struggles above 7 and close to 8.
I bought it to start learning about nd filters and whilst it s not as good as a true nd it's a good learning tool.
If you are looking at buying one filter them I would probably go for a 4. It really depends on when and how you are looking to use it though.
Canon 550D, Canon EFS 17-85 IS USM, Canon EF 75-300 IS USM, Canon EFS 10-22mm USM, Canon EF35mm F2.
Got some of the gear but really still no idea...