Noise at low iso setting is a main concern for me as I mainly do landscape photography. Problems with noise normally only becomes a problem when making bw pictures. Here the problem can be minimized by controlling the colour noise.
Anyway here are some tests showing optimum setting at low ISO. First a test with exposure times from 0.4 to 5 sec paired with ISO values from 640 down to 50 respectively.
The second test assumes that ISO 50 is the same as ISO 100 with 1 EV overexposure and again pulled back. The test compares ISO 50 with ISO 100, 200, 400, 800 and 1600 overexposed 1 EV and pulled back in Camera Raw 6.3.
The third test is equivalent, however, the RAW converter is Digital Photo Professional.

The fouth test shows the best setting if you want a fixed and fast shutter time for instance if you want to freeze movements of branches, grass ....

For all tests the RAW files where converted by Camera Raw 6.3 with all noise reduction disabled. Only the blue channel is shown as here the most noise is revealed.