While everybody has included China in their top five, I wonder how they could have missed India in there. Despite the fact that we have been invaded by so many different conquerors of different cultures, we have managed to assimilate almost everyone of them into our culture.
In the architecture and sculpture found in what is now Afghanistan, it is evident that Indian and Greek influences have long worked in Cohesion to produce masterpieces like the Bamiyan Buddha among others.
In fact, it was the Indian king, Ashoka who "exported" Buddhism to SE ASIA and the Far East and encouraged non-violence and learning within his own realm. Besides, this India was always at the forefront in Science and technology, esp in Maths, Astronomy, Metallurgy and Medicine.
The '0' the very basis for modern mathematics was invented in India, the Pythagoras theorem was postulated here much before Pythagoras by an Indian mathematician by the name of Boudhayana. In fact, several concepts of trigonometry and geometry were mastered long before the Greeks as the references to them in Indian scripture indicate.
In Delhi, there exists a pillar of iron which has not rusted for over 2000 years despite being exposed to the elements. In fact, King Richard the III was defeated by Saladin, who was armed with swords made from steel which the Indians originally cast.
In astronomy, Indian astronomer, Aryabhatta postulated that the Earth was round and went around the Sun once in 365 days, long before any Western Astronomer dared to think of such a thing.
I can go on and on, but I will not have come to the crux of my point. While China waas relatively fre from attacks from the Western civs, uptil about the 18th Century, India had to deal with the Greeks, the Persians, the Arabs, the Afghans, not to mention the French, English, Portuguese and the Dutch who followed!!!
Yet, our culture has remained steady and has been able to incorporate all the different ideas and views held by various other cultures who came as conquerors but eventually settled down and were assimilated into the culture.
Of course, we have been through our own problems, no doubt, the rigidity of the Bramhins, the petty quibbling of the Rajputs and the greed of the later Princely states have affected our culture and civilization no doubt, yet, we are now emerging from the ashes of colonialism and more recently, protectionistic economic policies, to take our helm as the forebearers of one of the world's most ancient civilizations.
I don't say I Indian is the best or the greatest civilization, but it is one among those whose stamp on history will be seen for years to come.
(Sorry for the rather lengthy post, but I joined this thread a little late
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But the fact remains that the reason the Brits could move in was 'cause the Indians had never been united enough to confront them effectively. 
