Which ND filter should i buy ?

Which ND Filter or any other filter is good to shoot long exposure image ? My lens is 18-140mm

There's no one-size-fits-all choice. It will all depend on the effect you want, combined with the lighting of the scene you're shooting.

A 3-stop is a good start, then you can expand from there based on your needs. Stick with the name brands and avoid the cheap crap on ebay and Amazon.

The best ND filters I've used are the X3 filters made by Breakthrough. Not cheap, but wow, they're good!

67mm X3 ND on Amazon

I tend to buy Hoya. I think they're the best bang for the buck.
In a pinch I've used the glass from welding helmets. I get them from the local welding supply store. There good for about ten stops.


https://en.wikipedia.org/wiki/Neutral-density_filter

Like @Zeke_M, I tend to go with Hoya for my filters. No complaints.

Zeke, a welding lens? Great idea!!