Pachacuti was the ninth Inca, and his rule is attributed by the people of Peru as what set the Inca on their way to greatness. It should be noted, however, that Pachacuti's story (and his accomplishments) is situated very strongly - first it was filtered by the Incas themselves, to make Pachacuti into the hero he was and then some; secondly by Spanish chroniclers that took down these tales as best they understood; and thirdly by modern historians and archaeologists. The chronicle of Betanzos especially is basically a heroic narrative of Pachacuti.
Pachacuti was not selected initially as the heir to the throne. He was the second son of Wiraqocha Inca, and known first as Inca Yupanqui. However, his rise to power came (as is often the case in these tales) in the midst of war. At this point, the Incas had established themselves over the whole Valley of Cuzco, more or less; however, they were struggling against their neighbors, the Chanka, to the west. The Chanka were fierce warriors, and were successfully rebuffing Inca attempts at expansion, which had recently taken on a more intriguing and diplomatic tenor as Wiraqocha Inca cemented alliances among the Lupaqa in the Titicaca Basin. The Chanka organized a powerful army to take Cuzco and destroy Wiraqocha's gains, and had shattered the ruler's armies as well as those of his heir, Inca Urqon. However, Inca Yupanqui refused to retreat and rallied the locals of Cuzco. The story goes that the Creator God revealed himself in a vision to Inca Yupanqui, and promised he would aid the Incas in their coming battle. Sure enough, the Chankas were shattered the next day by warriors who appeared to come from nowhere - the Quechua state that the very stones of the mountains rose up to help the Inca prince. Victories like this (and the loyalty of his captains from these battles) propelled Inca Yupanqui to usurp his brother Inca Urqon, and become the ninth Inca ruler, self-christened as Pachacuti, World-Shaker. Before leaving this part, I should note that scholars have found a great discrepancy in the timing and even the existence of the Chankas, depending on the chronicler - they could be a "mythical foil" of sorts, the "great enemy" seen in so many mythohistorical stories.
From here, Pachacuti embarked on the first major conquests of Tawantinsuyu - defeating the Qolla in Titicaca (and potentially getting all the way to southern Bolivia) and also embarking on campaigns to move northwest through the Peruvian Andes. Included in this meant subugation of Huancavelica, Cajamarca, and potentially the beginnings of the fall of Chimor, who were probably the "biggest kids on the block" for some centuries beforehand. In addition, campaigns of varying success ventured into the eastern lowlands, the Ecuadorian mountains and coast, and the southern Peruvian coast.
Back at home, Pachacuti is credited with drastic reshapings of the capital. It is said he redesigned Cuzco's central plan to fit between the two rivers of the upper valley, and designed it to look like a crouching puma. In the nearby Sacred Valley to the north, Pachacuti began construction on several royal estates - including Pisac and the most famous Inca site of all, Machu Picchu. In addition to these architectural accomplishments, Pachacuti also seems to have instituted religious reforms placing Inti, the Sun, as the primary deity of the Empire. This supplanted the "traditional" supreme deity, the Creator god Wiraqocha, who had one aspect known as Inti. These reforms allowed local deities of subjugated peoples to be respected and revered, so long as this was done second to the worship of Inti.
In all, Pachacuti (seen here in Cusco on a very Inca-looking, yet NOT Inca tower) is probably best compared as the Ataturk of Peru. He is credited with administrative, religious, and military reforms, and the instigator of the largest land empire in the Pre-Columbian Americas. Considering much of the Inca narrative is difficult to reconstruct, it is hard to say he actually was able to accomplish all that the Peruvians say he did. But at the very least, he was a very powerful figure in the Andes, and one certainly deserving of larger historical attention.
For the "colorful" version of Pachacuti's exploits, I suggest the Betanzos chronicle; otherwise Terence D'Altroy's The Incas from Blackwell is an excellent resource with archaeological context and multiple chroniclers' sources. D'Altroy's book was my resource for this answer. Pärssinen 1992, Tawantinsuyu: The Inca State and its political organization is also a good reconstruction of the empire's expansion and conquests.
