Dean -
You're going to get differing opinions from almost everyone, based on what his/her personal preference is. Generally, if you stick to one of the big 3 (Kodak, Ilford, Fuji) you're going to get consistent quality and predictable characteristics. There are also some less-expensive offerings from Foma, Kentmere and Arista, some of these being re-badged films from one of the big 3.
For B/W street photography, 400-speed films like Kodak Tri-X or Ilford HP5+ will be the most flexible and give you the classic B/W film look. In low-light these can easily be pushed to a higher ISO and still give quality results. I also use Kodak TMax400, which has a cleaner look (to my eye) and tighter grain. I believe Ilford Delta 400 is a similar type of film to TMax400, but I've never shot it.
For nature photography, especially Macro, I use slide films like Fuji Velvia or Provia (100) which give great color saturation. Not so good if you're photographing people. Also, slide films are less forgiving of underexposure, so you need to be accurate with your metering and avoid higher-contrast scenes. Color print films tend to be more forgiving, and Kodak Portra is pretty widely used - nice for general shooting and people/portrait photography especially.
For an idea of what a skilled nature photographer can do with 35mm slide film, check out the work of the late Galen Rowell. I believe he used Fuji slide films and he was an expert at using graduated filters in the field to overcome high-contrast scenes and get the look he wanted.