The first prozines I read regularly were Galaxy and Worlds of IF in the mid-to-late 1960s when Frederik Pohl was their editor. IF was mostly light fun, adventures that were more fun to read than lasting value. Of course there were exceptions to that, such as Robert Heinlein's The Moon is a Harsh Mistress and occasional short fiction as Roger Zelazny's "This Mortal Mountain" and Samuel R. Delany's "Driftglass." Galaxy printed the more serious fiction such as Clifford D. Simak's Way Station, Jack Vance's "The Last Castle," and Cordwainer Smith's "The Dead Lady of Clown Town." These stories combined all the ingredients I preferred in my science fiction reading: sense of wonder, exotic settings, thoughtfulness, and fascinating characters, all wrapped around a serious plot that I could sink my teeth into.
When Frederik Pohl quit as editor in 1969, both Galaxy and IF degenerated rapidly into clones of 1960s Analog: more technological stories, less exotic settings, more adventure plots. The exceptions were mostly their serials, largely because Robert Silverberg tended to publish many of his novels there, such as A Time of Changes, Tower of Glass, and Dying Inside.
By and large, my favorite prozine of the 1970s was Analog, where Ben Bova had broadened the scope considerably, to some extent resembling the Galaxy of the 1960s more than the John W. Campbell Jr. Analog of that period. He published a lot of George R.R. Martin, particularly Dying of the Light and "A Song For Lya," as well as Roger Zelazny's "Home Is the Hangman" and Robert Silverberg's Shadrach in the Furnace.
Another good prozine in this decade was Edward R. Ferman's Fantasy & Science Fiction. At that time it was the only major prozine which printed both fantasy and horror as well as science fiction, so perhaps one-half of each issue really appealed to my reading taste. But their science fiction was good: a lot of Michael Bishop, particularly "The White Otters of Childhood" and "The Samurai and the Willows", Robert Silverberg's The Stochastic Man (it seemed as if Silverberg was providing the highlights of all the prozines in that decade), and Richard Cowper's "Piper At the Gates of Dawn" and "The Custodians."
Then we came into the 1980s. I was not a fan of Stanley Schmidt's Analog since it had returned to the technological / adventure bent of the latter Campbell years, and Edward Ferman's F&SF was become stale and generally less interesting. The real action was taking place at Isaac Asimov's Science Fiction Magazine where Shawna McCarthy and later Gardner Dozois raised it above the simplistic adventures of the George Scithers years and were printing some very serious, high quality science fiction: Connie Willis' "Fire Watch," Greg Bear's "Hardfought," John Varley's "Press Enter," Octavia Butler's "Bloodchild," Roger Zelazny's "Twenty-four Views of Mt. Fuji, by Hokusai," Lucius Shepard's "R&R."
The problem, as I saw it, was that Gardner Dozois gradually broadened the scope of Asimov's too much. Not content with printing the best science fiction of any prozine, he also printed pure fantasy (mostly contemporary fantasy), horror, and even an occasional mainstream experimental story. So while Asimov's represented the cutting edge of science fiction in that decade, half of each issue was generally non-SF.
And that's still where the three prozines basically still are today: Asimov's prints the most important science fiction intermingled with contemporary fantasy and mainstream experimentalism; Analog specializes in technological, and adventure fiction; F&SF intermingles SF, fantasy, and horror.
Several other American prozines and semi-prozines have tried to fill in the gaps in the past two decades, but publishing being the risky business it is, none of them have maintained a high profile for very long. Algis Budrys' Tomorrow Speculative Fiction printed pure science fiction, mostly from new young writers, and tended to run the gamut of the genre. Science Fiction Age was a slick, multi-media affair, whose fiction started out weak and got stronger and stronger. Now there are a lot of irregularly-published semi-prozines, but nothing reliable (unless you go overseas to Great Britain where Interzone and The Third Alternative thrive artistically, if not financially). If prozines do die, I wonder how that will affect the state of science fiction? Semi-prozines are thriving, as are small press original anthologies and online publishing, but can they take up the slack to a larger, rather than lesser, extent? Stay tuned to these pages in another few decades for an updated commentary.