What was King Solomon's greatest contributions in the Bible?

If you are referring to the Israelite king, then his greatest contribution would be the inauguration of the monarchy in Israel, although he was a reluctant king at first. He had a number of military victories, but suffered due to a number of serious character defects.

Probably his single finest hour as King was the victory over the Ammonites in 1 Samuel 11, as well as his humble and peaceful attitude towards his opponents.

If you are referring to Saul of Tarsus, who later was known as Paul, his writings were undoubtedly his greatest lasting contribution. Some would regard the book of Romans as his greatest contribution.