Disclaimer: Everything that follows is just my opinion, based on my preferences, values and way of working.

I also like colours as near to natural as I can get. Portra 160 NC suits me well. I tried Fuji NPS as an alternative and found it to be quite similar. You could say that the NPS - NPH - NPZ and Portra 160 NC - 400 NC - 800 sets were all in the 'natural', wide latitude/low contrast category. NPZ and Portra 800 can be pushed up to two stops while keeping natural colours - but I daren't set my meter any higher than 1250 or 1600 with a two-stop push.

Choosing between them is a matter of taste. I find the Kodak films to be more closely predictable in their behaviour in mixed lighting - the Fuji films often do better, but sometimes fail miserably in mixed lighting (maybe I expect too much of them after reading all the fourth-layer hype). They all have low graininess for their speed - maybe Kodak still have the slight acutance edge which Fuji trade off for slightly lower graininess.

They all respond well to 'over-exposure': their curves carry on in a straight line a long way above the aim point for an 18% grey card, but only go down three and a bit (or so) stops to the toe. Some people set their meters a third of a stop under the box speed 'so they can get better shadow detail'. Set your meter a full stop under and you will get great shadow separation. In lighting conditions that are not standard daylight, over-exposure also helps to keep all the layers on their straight-line portions. Example: in household incandescent lighting - say 2800 K light - if you don't use a filter the blue layer could be 'underexposed'. Giving more exposure helps the blue layer without risking over-exposure in the red layer. I hope that I've explained that reasonably well. The point I'm trying to make is that 'overexposed' neg film gives you the best chance of having 'naturalness' in the final work.

Despite my preference for natural colour, I've just tested Kodak 100UC and 400UC. They are both impressive films with very wide exposure latitude. 100UC's toe lifted off at the same place as the nominally faster NPS and 160 NC in my side-by-side tests (admittedly limited to two examples of each film). I found that I could get natural results from 100UC, with pleasing skin tones.

Slide films: I used to use Kodachrome almost exclusively until '97 when I worked for a client who insisted on Agfa RSX films - which I'd never tried before. I found them pleasingly natural. Then Ektachrome E200 came along. I think that's a good all-round film that pushes and pulls well. E100G is in the same ball-park, but a bit more saturated. E100G has very low graininess. Good old EPN (Ektachrome 100 Professional) is still a standard in terms of accurate, natural colour for exposures shorter than about 1/8 (The 'E' family are better for longer exposures). I never really got on with Fuji slide films - always preferred the Kodak side of the acutance vs low graininess contest.

Best,
Helen