Allow me to show you some of the base meshes I've created. This library is very much a work-in-progress, so I would appreciate comments and suggestions.
As a starting point, I believe every CG artist needs a "parts" directory, with eyes and ears and such. Here's a stock eyeball with a simple procedural texture map:
Notice it's two meshes, the transparent (but reflective) cornea over the glossy iris/pupil layer. Use a ramp texture with stretched fractal noise mapped to the iris band. The "color" of the eye can be changed pretty quickly by changing the color of this noise texture. Looks good enough for 90% of CG jobs. For game engines, leave off the cornea sphere and just use the inner eyeball.
Next up we have an ear. Keep a few ears around; they are laborious to model. Import and attach a stock ear whenever you can; it could save an hour of development time. I have more refined ears, but I find a simple block outline of an ear (above) to be the most flexible. It allows me to get a decent head-start while leaving me free to model unique characteristics (ears, like noses, are highly individualistic).
Next you need a good set of choppers. That is, every character needs at least teeth and gums. They show up in almost every facial expression. Primary characters also need a tongue, to form phonemes and create more refined expressions. You should keep a few levels of detail of teeth around. The most extravagant should have individually modeled teeth, tongue, and mouth roof (above). The next level down will be the most useful: a simple set of teeth and gums mindful of poly count. Great starting point for most characters.
At the next level you should have a very simple two-dimensional set of teeth for background characters. At a distance, they won't be revealed in the round, so just model the front side of the teeth. This one has around 900 tris:
Next you need a good hand. Most of the time I go ahead and model the hand to suit the individual, but I keep a stock of generic hands around just in case I'm running out of time, like this quickie I recently built:
Next you need a few heads. Facial topology takes the longest to refine, so any time savings here is greatly welcome. Once you hit upon a good working topology, it can be morphed to create any number of characters, male or female, merely by pushing groups of points to change proportions. Below is an early one I've used in a few projects, inspired by the Osipa method. It has a fairly decent nose, mouth and eyes. The ear is pretty ridiculous, but it was meant to be obscured by hair so it was never really developed.
Next up, you should have some seamless full-body base meshes. At least one male and one female. I've been working on females recently so that's what I'll be focusing on. Here's a simple generic one:
Then I pushed it to create a more stylized version:
I pushed things even further with this highly stylized hero-woman base, employing the technique of extruding individual muscle groups to create a lo-poly topology that defines exaggerated form with minimal geometry:
I've also been working on this more "realistic" base for female characters that aren't stylized:
Also, I keep a lot of character primitives around as a starting point for cartoon archetypes. Like this guy: