Analog is the longest running continuously published science fiction magazine. It started in 1930, and is still published today. Authors featured in Analog have included Isaac Asimov, Robert A. Heinlein, HP Lovecraft, Orscon Scott Card, and many others.
Galaxy ran for 30 years, from 1950-1980. It published some of the first stories of many notable authors. It was Galaxy that first published a short story by Ray Bradbury called "The Fireman" which was later expanded into the classic novel Fahrenheit 451.
IF: Worlds of Science Fiction ran from 1952-1974, when it was merged with Galaxy. Some notable authors who published stories in IF were Harlan Ellison and Larry Niven.
The Magazine of Fantasy & Science Fiction was first published in 1949, and continues to be published monthly today. Isaac Asimov wrote a science column for the magazine that ran for 399 monthly issues without a break, from November 1958 to February 1992.