In terms of both talent and career achievements, Nastase was clearly the best out of those players. Smith isn't that far behind in second place and the two of them had a good rivalry. However in addition to Nastase's greater volume of career titles and major indoor titles, he was more versatile as he was far better on grass than Smith was on clay.
Kriek has to be bottom of the list given that he won his two Australian Open titles during the tournament's bleakest period. Still he was a pretty fun to player to watch when he was playing well.