What an interesting question. It is, of course, a bit on the subjective side. All of the four ideas are important, and depending on your situation, you might find a different one to be more important than the rest. Let's look at them all to see if we can come to any conclusion, shall we?
- Truth -- Well, how could this one not be important? Any philosophy must be based on truth, and the action of every person (if they are to be genuine and helpful) need to be based on principles of truth as well. If your thoughts and actions are based on falsehoods, everything else is destined to fail. Gandhi attempted to search for truth by learning from his mistakes and trying to conquer his own demons.
- Non-violence -- Gandhi said, "When I despair, I remember that all through history the way of truth and love has always won. There have been tyrants and murderers and for a time they seem invincible, but in the end, they always fall — think of it, always." Non-violence was a central part of his philosophy and is very important to his message. Without the non-violent approach Gandhi's movement might have ended in a bloodbath and we might remember him very differently.
- Renunciation -- This is a hard one for the Western mind to grasp. It is, essentially, "desirelessness." It is the training of the mind to renounce power, and want, and greed, and other mortal desires. How can a person be truly free if they are slave to these forces? It is a major problem for anyone who leads a movement...there comes a point when that person doesn't want to give up control of the organization. It is a very human force, but one that needs to be eliminated in order to live a more pure life.
- Self rule -- This is another concept that is easy for us to dismiss because it is foreign to us (in the way that water if foreign to fish because they never think about it.) "Self-Rule," taken generally, was Gandhi's way of saying that the Indians should be able to choose their own path without their British overlords. He wanted to eliminate this force on Indian life. Taken more specifically, Gandhi felt that each person should have "self rule" over their own life and have the power to choose their own destiny. Who could argue that this wasn't an important idea?
So there you have it. All of the ideas are important, and all coexist with the others. You'll have to decide for yourself if you feel that one of the ideas is more powerful than the rest. As for me, the question is like asking which tire on a car is most important? Without all of them, you aren't going anywhere.
Among all the ideas supported by Gandhi and used by him to lead and guide the people of his country (India) and rest of the world, the concept and practice of Non-violence is the most important. He used this ideas intensively and extensively in his approach to public action, as a philosophy of life, and a way of life for himself.
The next in importance, according to me is truth. But I must say that Gandhi himself asserted truth as the most important. He claimed his life to be a constant search for truth, and therefore, titled his autobiography as My Experiments With Truth. In placing, truth, next to non-violence rather than before it I am not contradicting the value placed by Gandhi on truth. I am only saying that the non-violence is more prominent, as compared to truth, in teachings and programs of Gandhi.
I am not sure if renunciation is a central feature of Gandhi's approach. In a way, Gandhi opposes, rather than supports renunciation when he asked people to struggle for their rights rather than just be satisfied with whatever the fate has to offer.
Self-rule is a political goal pursued by Gandhi for India. In contrast non-violence, self-renunciation, and truth are policies of self conduct and political action that guide the action for achieving self-rule and other political objectives. It must be noted that individuals may shift from new goal they achieve the existing ones or find them to be no longer appropriate under changed condition. However the basic policies remain unchanged under all conditions, Therefore the policies are more important than any objectives which may be achieved by application of those policies.