By far the most riveting play recently on view in an otherwise rather lacklustre British theatrical season has been the Royal Shakespeare Company's Macbeth. Trevor Nunn's production opened originally in Stratford at the RSC's tiny, experimental The Other Place in the summer of 1976, and transferred a year later to the company's equivalent London theatre: The Warehouse, a small, recently renovated auditorium in Co vent Garden. The Warehouse is aptly named: a high-ceilinged space, with three rows of seats on two levels surrounding the playing area on three sides, it has probably the worst sight lines of any theatre in London. But what it lacks in architectural excellence The Warehouse gains in intimacy (seating capacity: 190). With practically no physical distance separating actors and audience, and played straight through without risk of the kind of emotional distance provided by an intermission, the RSC production creates the most intense and powerful Macbeth I have ever seen.