This one will not be easy to answer and will have many different interpretations. If we are looking at the first personal computer that was not intended to be housed in large offices for large scale business, I would say that the Altair 8800 developed by Ed Roberts from a Southwestern Computer Company called MITS would have been the first personal computer that began the computer revolution. It was debuted on the front cover of the January 1975 issue of Popular Electronics. The Altair was a personal computer "kit" which made the computer more accessible to individuals, as opposed to the commercial and industrial computers being used at the time. The reason why I cite this particular milestone as the invention of the computer is that a few months after the Altair model had been made available, two young computer programmers named Bill Gates and Paul Allen developed the first BASIC computer program that could be run on the Altair. After the Altair proved the openness of the computer market, other programmers started to take steps to develop personal computers, such as the Apple I and Apple II from Steve Jobs and Steve Wozniak. The emergence of Gates and Allen, and later, Steve Ballmer, as well as Jobs and Wozniak permanently transformed the industry and began the computer revolution in which we live today.
A lot of computing machines were being used for computing activities. Some of the early known recorded facts are:
Pascal's mechanical calculating machine in 1642.
Gotfried Willhelm Leibnitz's calculating Machine in1672.
Charle's Babbage's Universal Electronic Calulator in1812.
Charle's Babbage's analytic Engine/Automatic calculator in 1832.
H Holebrith 's Computer (Tabulating Machinery) in 1889.
A series of inventions of electronic components and the digital algebra has contributed to the computing history during the 20th century.The increase in the speed of processing , the amount of memory and the storage capacity besides making it more portable have become the main issues now.