IF, BY THREATENING to hang me from the Sam McBride's highest yardarm, somebody forced me to single out the best attraction at the eighth annual Mariposa Folk Festival, I'd have to say that the real star was the setting.

After the dust and mud of Innis Lake, the festival site in past years, Toronto Island presented almost an idyllic setting for the three-day show. There was a feeling of freedom and spaciousness about the island site that Innis Lake never offered in spite of its being in a rural setting. Maybe it was all those roadblocks and cops who used to frisk any suspicious bundles and women's handbags for booze.

This year, by contrast, there were a handful of Metro policemen and a few special guards on duty and the only official word on booze was a small sign saying, "No liquor." But no frisks, no hassles and no drunks.

With Toronto's skyline visible across the harbor, there was also a feeling of being with it and, at the same time, of being removed from the irrelevancies of big-city life.

With its weekend sailors out in force and the ferryboats plying back and forth, the harbor provided constantly changing backdrop for the festival. On Saturday afternoon, during the blues workshop, there also was the intrusion of real-life drama as a sudden high wind capsized at least three sailboats within sight of the audience.

As for the concerts and workshops, there was a heady atmosphere of success and well-being instead of the usual feeling at Innis Lake that the whole production was anything from a rural slum to a disaster area.

So, with no really big names, a well-organized, well-balanced program and, above all, the Island, it was clear sailing all the way. According to an unofficial estimate, festival attendance passed - by approximately 3,000 - the 12,000 total necessary to break even.

In short, it was the year the Mariposa Folk Festival came of age and the wonder of it all is that nobody thought of using the Island before this. With the obvious success of the festival as an indicator, Metro Parks Commissioner Tommy Thompson, Metro Council and interested parties should think seriously about building a permanent open-air concert bowl there which could be used not only for the folk festival but for classical music and jazz as well.

ARTISTICALLY, the festival was an unqualified success if only because it was totally true to the meaning of the word folk. By contrast, the Newport Folk Festival - to give the gate a shot in the arm - usually manages to shoehorn in a few acts which could never, by the wildest stretch of imagination, be regarded as folky.

At Mariposa, while it would have taken the most catholic taste imaginable to be wild about everybody who appeared, there never was a real dead spot or feeling of letdown in the concerts which were kept moving by holding encores to a bare minimum.

And at the daytime workshops I managed to observe, I got the feeling that the performers appearing were sincerely interested in educating and informing their audiences about the various aspects of folk music rather than just performing at concerts-in-miniature. One of the few exceptions, the Saturday afternoon blues session, was in fact a small concert but, because it featured Howling Wolf and his Chicago Blues Band and blues singer Bukka White, both of whom communicate best through their music, nobody was complaining, least of all me.

It almost seems unfair to single out any of the performers who appeared but these are some who stick in my mind:

Oscar Brand presenting his respectful but not worshipful tribute to Woody Guthrie in which he listed a few of Guthrie's political changes of heart - particularly his complete about-face on World War II after Russia entered it - just to show that while Guthrie may have left a legacy of great Americana, he was only human;

Frank Wakefield the mandolin player of the Greenbriar Boys, who came to the festival solo to appear in workshops, managing to get invited onstage at all three evening concerts, much to the delight of the audience;

Bukka White telling his predominantly teenage audience that "you do't know anything about the blues, but us older folks do," and proceeding to prove it with such songs as Stuttgart, Arkansas, and My Mother Died When I Was Four Years Old;

Howling Wolf knocking out everybody - and almost himself - with an almost unbelievable rendition of Goin' Down Slow;

Mike Cooney proving time and time again that he practises what he preaches - it's the song and not the singer that counts;

Mike Seeger showing a dazzling display of virtuosity by playing in turn the fiddle, banjo, guitar, mandolin and autoharp and singing most of the time;

David Rea getting one of the biggest ovations with his own compositions, The Tupelo, Miss., Flash and David and Goliath;

Joni Mitchell proving anew that she has few superiors when it comes to imagery and vocal portraiture;

Jim McHarg's Metro Stompers representing the bridge between folk music and jazz with their usual sterling performance on such tunes as Panama Rag, Just a Closer Walk With Thee, St. Louis Blues and Do You Know What It Means to Miss New Orleans;

Gilles Vigneault, the Quebec poet, composer and performer, presenting his almost unique brand of music and, at times, reminding me of nothing less than a French Danny Kaye;

Bill Monroe and his Bluegrass Boys playing their freewheeling and, at the same time, carefully orchestrated music;

Steve Gillette, a glib, Southern California version of Gordon Lightfoot, performing his own Molly and Tenbrooks and Darcy Farrow;

Vera Johnson coming on like a folksy Mae West with a composition of her own called I'm All Woman and a Yard Wide;

The Young Tradition, an English group, sounding like time travellers from the past with their haunting recreation of old ballads and carols.