Kalam was a scientist. If we speak hardly his research wasn't ground breaking. But he is a symbol of hardwork and preserverance. More credit goes to Sarabhai for trusting him to the core. That's what leaders are made of.
As for the title, it would be a match between Patel and Ambedkar. Both undertook missions and decisions that would have long positive implications even though there was opposition inside and outside the party.
If not for Gandhi interference, he would have been our first PM of independent India.
Nehru also did good as he kickstarted Indian policy of Panchasheel, non alignment and gave a federal set up strong for its time.