Who was elected US president in 1884?

This depends on the country/organization. See related questions.

US: Rutherford Birchard Hayes, until March 4. James Abram Garfield from March 4 to September 19. Chester Alan Arthur after September 19.

Rutherford B. Hayes was the 19th President of the United States from 1877 March 4 to 1881 March 4. He was previously the Governor of Ohio.

James Abram Garfield was the 20th President of the United States from 1881 March 4 to September 19, when he was assassinated.

Chester Alan Arthur was the 21st President of the United States from 1881 September 19 to 1885 March 4. Due to health problems, he did not seriously run for a second term. He would die a year and a half after his presidency, on 1886 November 18.