TS Eliot's The Cocktail Party premiered at the Edinburgh Festival nearly 50 years ago and this week it returned. But unlike the author's more famous Cats, this show only ran for eight performances. A shame, because Philip Franks's revival deserves a longer life. As with Stephen Daldry's revival of An Inspector Calls, Franks's enthralling production reintroduces us to an accomplished mid-century dramatist - and, in this case, to one who thrills us with his intelligence. The Cocktail Party works as well today as ever.

It seems rooted in the recognisable milieu of the well-made play. There's a party in a London flat and the host's wife has done a bunk. Among the stranded guests, Maggie Steed is majestic as the blowsy, husky, socially domineering Mrs Shuttlethwaite ("I never probe," she says).

Simon Jones is the picture of posh affability as Alexander Gibbs. Only Clive Merrison - at this point the Unidentified Guest, later revealed as the psychiatrist Sir Henry Harcourt-Reilly - has a manner more graveside than bedside. In a striking performance, Merrison sits as motionless as a cardboard cutout (with glistening bald head and coat-hanger figure) taking a beat before replying to any question. Later, he displays a piercing, merciless intellect.

Tension animates the play: not just between the married couple, or the couple and their lovers, or the host and the guests, or the psychiatrist and the patients. There is a constant stylistic tension too: between the conventions of the well-made play and the possibilities of verse drama; between the spinners of cocktail badinage and the author of The Waste Land.

Three of the characters are "guardians", guiding the Chamberlaynes and their lovers on different paths to salvation. David Bamber is excellent as Edward Chamberlayne, the perplexed, stymied barrister ("You talk as if I was capable of action"), a podgily dappy figure waving his anguished arms, as if perpetually brushing away a sluggish wasp. As Lavinia, with red lips and nails, dragging hard on her ciggie and bickering with her husband, Suzanne Burden is as impressively brittle as the vase she smashes. Catherine Cusack has exactly the right Forties look as Edward's lover, Celia, and brings a lucid frankness to her spiritual quest. And as Peter, the Hollywood screenwriter, Sebastian Harcombe has a handsome intensity that suggests he will be up for the next BBC costume drama.

Franks's production is exemplary: with Rae Smith's semi-abstract set, Matthew Scott's music - ominous, elegant and brief - and Howard Harrison's lighting, which steers the drawing-room characters into alienating whites and hellish reds, then pins them down during the heated interrogations in Sir Henry's consulting room, with shafts of light. Eight performances of this is not enough.

At times, in the three and a half hours of Peter Stein's production of The Cherry Orchard, we could have been moving with the same slow careful steps of Branko Samorovski, who plays the ancient footman, Firs. Samorovski's ghostly head sinks so low that his nose looks closer to the ground than his chin. He fusses and pampers his master Gayev (Peter Simonischek, a warmly phlegmatic portrait). Firs takes an age to cross the stage and we are with him the whole way. As the Edinburgh lady in my row said: "It's certainly an experience."

First produced in Berlin in 1989, Stein's German-language Cherry Orchard was restaged at Salzburg in 1995: now it arrives in Edinburgh for a series of farewell performances that bear the hallmarks of its marathon gestation period. Imagine a large oil painting touched in by a miniaturist. Events here don't bump up against each other with the unexpected rapidity that occurs in less considered productions. They unfold with the social complexity of an 800-page novel. You daren't shoot more than a glance at the surtitles in case you miss the next detail. Karl-Ernst Herrmann's spacious set uses few furnishings: the orchard stands on the same planks that form the interior floor. The trees are on a slant, suggesting they may slide off at any moment. Stein takes this ample space and fills every inch of it.

After the businessman Lopakhin (Daniel Friedrich) announces that he has bought the orchard, he leaps round the drawing room and crashes into the double doors. The candles fall off the wall. He catches them mid-air. Then he thinks better of his instinct and hurls them to the ground. As he does so, Madame Ranyevskaya (Jutta Lampe), who has been sitting centre stage witnessing his antics, hurls herself to the ground too.

The third act is a virtuoso piece of directing. Stein stages a dance, with the guests and Jewish band visible through three double doors while the main action occurs in the room downstage. We also hear people playing billiards off-stage. Unnamed characters drift in and out at awkward moments without any sense of falseness. Stein creates a world that extends far beyond each scene.

Each character is made richly individual. The arrogant footman Yasha (Roland Schafer), with his vain flop of dark hair and malevolent strut, can't resist looking bored when the subject of Madame Ranyevskaya's dead son comes up. The maladroit estate clerk Yepithodov (Gotz Schubert) has hilarious problems with inanimate objects. We keep expecting him to hurt himself, and he keeps surprising us by the way he does. In the slow first act, when Madame Ranyevskaya returns at 2am after five years away, the characters' reactions might have been scored as a symphony of yawns, sighs and snores counterpointed by outbursts of high emotion. The reaction is appropriate: this Cherry Orchard is a huge experience, and also quite draining.