On an 11-16mm ultra-wide, a slim circular polarizer is the safer choice because standard-thickness filters are more likely to vignette. A slim CPL can work well on lenses in this range, but stacking another filter on top usually increases the risk of vignetting, especially at the wide end.

For graduated ND filters, a square/holder system is generally more useful than screw-in grads because you can slide the transition up and down to match the horizon. Screw-in graduated filters are limiting since the gradient position is fixed.

Cokin graduated filters are usable, but users often report some color cast. If you want more neutral results, Lee-style grads are generally regarded as better, though typically more expensive. Cokin P and Lee filters can be interchangeable in some holder systems, but holder costs may differ.

For landscapes and waterfalls: use a slim CPL if you need one, avoid stacking on the 11-16mm when possible, and consider a square graduated ND system if you want flexible horizon placement.