Plato and Aristotle.
Aristotles philosophy was basically the opposite of that of Platos. They disagree in virtually everything, which I found very amusing considering that Aristotle was Platos student.
Plato believed that reality is permanent and our senses cant be trusted. He believed that ideas are eternal, self-contained absolutes, which answered to each item of exact knowledge attained through human thought. Ideas are in Plato's view concrete standards by which all human endeavor can be judged, for the hierarchy of all ideas leads to the highest absolute - that of Good. He also believed that Ideas are immortal and are more real than the mortal. Aristotle on the other hand believed that for something to be real it had to have a substance and a form or a body and a soul. Our senses are also reality. Aristotle refutes Plato's belief that Ideas are perfect entities unto themselves, independent of subjective human experience. Ideas, Aristotle claims, are not abstractions on a proverbial pedestal but mere duplicates of things witnessed in ordinary daily life. Thus, according to Aristotle Ideas are not perfect entities, indefinable to subjective human experience, but originate somewhere in ordinary human activity and perception.
Their views on afterlife were quite opposite as well. Aristotle believed nothing happens when we die because to even exist you should have both a body and a soul. So if you die and your body leaves then you dont exist because you dont have both a body and a soul. Plato believed in some kind of reincarnation. He claims that states of being are contingent upon the mingling of various Forms of existence. When one dies, that only means a death of a material part or ones physical body. To Plato its not important as soon as we have souls or our immortal parts. Later on your soul can inhabit another body and then another and so on up to eternity. In a sense you kind of get recycled over and over again.
The question about Gods existence and nature differ as well. Though both of them agreed to some extent to the existence of God but according to Plato's philosophy God exists as a supremely good being whose goodness is analogous (but not identical) to his Idea or the ultimate good. He owes the existence of his Ideas to both God and goodness, but he claims the two are not the same. It means that God is not completely perfect, but God's intentions and actions have good aims - goodness may emerge from other sources besides God. God, according to Platos philosophy appears to be not an ultimate creator but a coworker of goodness. To me this eliminates the whole meaning of God as a being conceived as the perfect, omnipotent, omniscient originator and ruler of the universe. Aristotle on the other hand argues that God is a self-sufficient, compelling force for both nature and man. He believes that all goodness comes from within God and that the goodness in man is drawn toward God and nothing else. This idea is quite consistent and would serve as a good explanation from a pre-scientific point of view.
In summary, Aristotle and Plato indeed hold quite opposite views. But interestingly enough I found that their answers to philosophical questions strangely compliment each other. I would agree with most of Aristotelian philosophy, which to me is very close to that of a modern scientific prospective. But in concern with political and sociological considerations I lean mostly towards Platos views. His idea of Utopia is quite appealing, although certainly not perfect. Not to mention the Aristotelian views of women as something inferior and incomplete I, as a woman find very offensive.