Obviously the Indian National Congress.. And don't you dare believe that it is actually Congress (I).
The INC has been a true secular party for more than 100 years. It has the maximum contribution in freedom struggle. Its greatest leaders are Mahatma Gandhi, Sardar Patel, Jawaharlal Nehru.
Whenever you have doubt on any Congress candidate's secular credentials, character or capability, just close your eyes, remember the "true" congress leaders, and cast your vote in his favor.
Even Mahatma Gandhi wanted each and every Indian to do exactly the same. In his various books he has explained how Congress is the only true party that he particularly wanted to rule India forever. Its another matter that some selfish people with vested interests killed him and falsely put it in his books and speeches that Gandhi wanted Congress to be disbanded after 1947. In fact they also removed all references from Gandhi's books and speeches about Sonia Gandhi, whom Mahatma described as a symbol of true World Unity - and he urged every Indian to cast their vote in her favor to bring India and Italy together.
So you please don't ask stupid questions like naming secular parties.. :-)