What is the best antibiotic for upper respiratory infection in pregnant women?

The correct answer is actually none. Many studies have shown that prescribing antibiotics does nothing to shorten the length or severity of symptoms, regardless of the color of the sputum, etc. The only studies that have shown any benefit (and there are only a few, compared with many studies showing no benefit) showed a very minor difference, for example a day less of symptoms. And for that very questionable benefit, you get the development of resistant organisms which are quickly becoming a public health crisis. Look for headlines involving "MRSA" or "VRE" for more info on that. Also look up "C. Diff Colitis" for another example of the dangers of overprescribing antibiotics.