Whomever was closest and met the criteria. Hal was simply in the right place at the right time, if Guy Gardner had been closer to Abin Sur then the ring would have undoubtedly chosen him.

As a side note, the rings also appear to posses a rudimentary intelligence and ability to read a situation so a worthy surgeon in the middle of a life-saving operation might be skipped over in favour of someone who wouldn’t be impacted by suddenly being offered it. The rings likewise will also not force themselves onto somebody against their will unlike some of the other corps. At the end of the day if a situation is urgent enough a ring will seek out the best candidate among those available in the immediate area to deal with a continuing threat, or simply return to Oa for the Guardians to decide if it decides that is the best course of action to take. It should be noted that lanterns can nominate their own (temporary or permanent) successor as well in certain situations but this doesn’t happen that often as it takes a lantern of exceptional will to override the default programming of the rings.