can anyone tell me which is the best swimming pool in N/K??

Of the ones I have found and tested, I would have to mention the Nong Khai resort pool, it is fairly big and deep enough for small dives.
It's also clean and well maintained, and has a kids pool also adjoining the main one.

The price is 60 baht for adults and 30 baht for children.

There is a restaurant and bar on the premises if you need a drink or a snack, both are pretty well priced.

I have attached a couple of photos.

It is located near to Nong Khai train station, Map ref: E6





There is also a good pool at the T19 resort, a few km's from Nong Song Hong on the main road to Udon, they also have a bar and restaurant:

If you are looking for best swimming pool at Nong Khai, the Nong Khai Resort is the place to be. They have a nice pool, especially if you try to eat on restaurant.

I am fully agreed with you. As I can see the "Nong Khai resort" seems the best resort in there. Not just that, the pool is very clean and beside it, have many green plants which add the view of it. Even though I am not been there, I feel the fresh air touching the skin of every people who visit the area.