One of the most frequent questions I've been receiving lately has been on why I only use Portra 400 for all my C41 work. Everyone seems to have a favorite and I often get suggestions that I should really try brand xyz, or try slide, or try this or that. I am often "informed" that I should be using different stocks for different scenarios depending on light and subject. But I disagree. I feel I should use what gives me the desired results, and Portra 400 gives me the look and feel I desire for all situations, from architectural work to portraits, landscapes to street. It's become a part of my signature look - and the way Richard Photo Lab works with it just blows me away. What I really enjoy about this stock is the vibrance (or lack of extreme vibrance) in the color of this film. Very real, very pleasing tones for my eye. I also love the latitude. I can expose Portra 400 (within reason) all over the place and still get great looking images. The more I expose, the warmer the tones become without killing the highlights or shadows. The closer to box speed, the cooler the tone. See this post by Johnny Patience on what I mean about warmer and cooler via exposure. I don't want to re-invent the wheel here, and he describes it perfectly.
My two favorite C41 film stocks? Kodak Portra 400 and 400 NC. Yes, I have tried many many other stocks - and some I really do enjoy, but not nearly as much as Portra 400 / NC. Right behind these is Portra 160 / NC and then I go to B&W for either Kodak Tri-X or Illford Delta.
I'll let a few photos in different environments explain what I like: