Base ISO is the optimum ISO for the camera - the ISO with the best overall image quality expressed as a whole stop. That means either ISO 100 or ISO 200 are the choices for base ISO. With one of the recent cameras, the best ISO was actually at Lo 0.3 - but it would never fly from a marketing standpoint and was not published as the base ISO.
Typically ISO levels are rounded and actual measured ISO may be as much as plus or minus 1/3 stop at any given level. The actual ISO levels are almost never the selected values. As you would guess, this means two different cameras might have different exposures at the same settings. DXOMark and Bill Claff have published measured ISO data for each stop.
On the low end, performance typically is still very good at using the Lo ISO levels. The optimum performance varies slightly by camera, but with almost all bodies Lo 1.0 is better than one stop above base ISO. You can freely use Lo ISO levels with minimal adverse impact. I find that for stream photography, Lo 1.0 with a circular polarizer is a very good alternative to a ND filter. I typically use a Lo ISO setting when I want a long exposure even if I add filters.
On the high end maximum ISO is more marketing hype and the highest level Nikon says is acceptable. The high ISO levels are certainly not optimal - they are last resort levels with considerable noise and much weaker color and dynamic range. Hi ISO levels are viable options as a last resort, but generally mean you are willing to trade image quality for shutter speed. I will give Hi 1.0 a shot if I have no choice - but it is truly a "Hail Mary". I might get something good - and its better than no image at all or subject blur.