IT WAS AN invitation no book reviewer could refuse. Several weeks ago, well in advance of the pre-Christmas turmoil, I and a dozen other regular reviewers of fiction for Book World received an invitation to participate in a holiday symposium on this tantalizing question: "Which character in fiction would you most like to be, and why?"

Not in the least surprisingly, none of those offered the chance to participate in this novel seminar declined to do so. For some, as you can see from the pieces below, the choice was easy; for others it was difficult, torn as they were among a number of characters who had managed to work their way into their hearts.

NO ONE classy, certainly. Too much intensity and high-level angst. But no one really . . . well, really low-level, if you see what I mean. And no one actually middling, either. The middles of things are seldom the most interesting.

But who, then?

Well, all right. I confess.

Ever since I was a kid -- and since, at 41, I'm still only aimed in the gereral direction of maturity, I figure I still have a chance -- I've always wanted to grow up to be Simon Templar.

Since Leslie Charteris first created the Saint -- tough and witty exponent of goodness and honor, defender of the downtrodden, righter of wrongs, lover of many beautiful women, charmer extraordinaire -- the fellow hasn't aged a day, not even through various incarnations on film in the persons of George Sanders and Roger Moore.

Failing that, I suppose I could make do with being one of the Hardy Boys -- the taller one -- but I have a feeling he doesn't enjoy all the advantages of suave and hard-fisted maturity. No, I'd really rather be the Saint.