At the centre of The Architect by David Greig, one of Scotland's most exciting new dramatists, stands the isolated figure of Leo Black (superbly played by Alexander Morton): architect, husband and father of two. The residents of Eden Court, the estate he built in 1971, want to blow it up and replace it with community housing. When he points out to the indefatigable campaigner Sheena Mackie (a perfect mix of friendliness and aggression from Una McLean) that the award- winning tower blocks were designed as a circle "like Stonehenge", she replies that no one was expected to live in Stonehenge.

The idealistic Morton believed in model buildings just as he believed in model families. His wife and two adult kids teach him otherwise. Morton gives the architect a grim, aching reality. He has cropped hair, a creased forehead and crumpled emotions. He contends with a peculiarly middle-class form of loneliness: professional despair.

Simon Vincenzi's set of the wooden skeleton of a room neatly stands for Morton's home, the tower block, the building site or the inside of a container lorry. Around it, Greig creates multiple plot-strands, intercutting from one to the other, as if four low-budget movies co-existed in one stage play. The blonde hitchhiking daugher, Dorothy (Ashley Jensen), develops a rivetingly tricky friendship with a bulky, candid lorry- driver (John Stahl). Meanwhile the seethingly sarcastic son, Martin (a pointedly clipped Tom Smith), goes cruising in the gents and picks up Billy (the vibrant Paul Hickey), a reckless hedonist who walks on the edges of high-rise buildings swigging creme de menthe.

With only seven characters Greig suggests a multilayered contemporary world. In this complex tangle, each character shares Greig's sharp eye for discerning unwelcome truths. The father eyes his daughter longingly as she lounges in the sun. The eerily sanitised wife (a cruelly monotone Morag Hood) views her husband as a hostage would view his cell-mate. The gentle lorry-driver tries to unravel his complicated feelings for Dorothy over cups of tea in a service station.

Greig has clearly been influenced by Ibsen. He ambitiously blends public and private concerns in a tight dramatic structure. But there's another echo: in the gap between familiar bourgeois comforts and the stranger world outside, in the massive gulf between parents and children, and the fragile alliances people discover in modern urban life, Greig also provides a thrilling rival to Stephen Poliakoff.

Moment after moment sticks in the memory. The writing is chilling, astute, assured, and the cast deliver exact and painful performances under the impressive direction of the Traverse's incoming artistic director, Philip Howard. One of the best plays of the year.

If the "work play" is a recognisable type, so too is the out-of-work one. In it, people not only have the time to make big speeches, but others have the time to listen to them. In Shining Souls, a very funny city comedy by the Glasgow writer Chris Hannan, the guys doing the soul-searching are the ones who've blown their Giro on the dogs. In this abrasive atmosphere, when you look for a shop to rob "the lack of choice is a scandal". Hannan articulates fiercely felt, and often hilariously illogical emotions. His sharp ear for Glaswegian distinguishes "achs", "ochs", "uchs" from "uh-uhs", and sends his characters off on richly demotic arias of self-justification.

Ann, an enticingly attractive Alison Peebles, has agreed to marry two guys called Billy on the same day. One Billy is a neat, lumbering type (John Stahl, again), who admits that he may not be clever but he can lift heavy things. The other Billy, smaller and bouncier, maintains the fact that Ann slept with the other Billy last night doesn't mean a thing. "I've decided I can't make a decision," Ann says, bluntly; she likes the look of Charlie (Stuart McQuarrie). He arrives at the market in search of a suit so he can visit his mother, who isn't his real mother, who is dying in hospital. McQuarrie can work himself into a sublimely indignant froth over trying to borrow some "dinaro" from his ex-wife or passionately defend his whingeing friend Max, who he admits is a lying bastard.

The drawback with Shining Souls - apart from the pale sense of place evoked in Ian Brown's relaxed farewell production for the Traverse - is the clash of tone. It's not easy to laugh at the two Billys ludicrously displaying their love for Ann by carrying a huge wardrobe back from the market, when we know that this wardrobe will replace the smaller one in which, nine years ago, Ann's two sons, Robert and Martin, apparently hanged themselves. It's a clash of genre, really.

After the pleasure of encountering contemporary Scottish life convincingly dramatised by younger playwrights, it's a disappointment to find John McGrath creating a large-cast production for the Festival that is nothing more than a weak satirical pageant. He has taken the medieval Satyre of the Thrie Estaitis by Sir David Lyndsay - a long-time Festival favourite - and added one more estate, the media, to transform it into A Satire of the Four Estaites.

In the two and three-quarter hours of elementary verse, each character gets a fulsome introduction before the play itself actually begins. That's after the interval. First we encounter, in the unlikely setting of Edinburgh's new conference centre, a mock-religious procession with dummies of Margaret Thatcher as the Blessed Virgin Margaret, John Major as St John the Apologetic and Tony Blair as Jesus Christ. At one point Blair is described as "a saint indeed / canonised without a single deed": the unadventurous rhyming is not untypical. "Offence" rhymes with "fence". "Platitude" with "latitude".

The targets are dull stereotypes. It isn't easy to satirise a Page Three girl, as McGrath tries here with Gloria Cupsize (Harley Loudon). The original is itself a caricature. The Lord Rees-Mogg figure, Sir Righteous Denunciation (Roland MacLeod) slurs his words and complains about "shoaps and shitcoms" on TV. There is also a Lord Merde ("och!" go the others). If the real Rupert Murdoch were as moronic as John Bett's portrayal suggests, no one would have anything to worry about. In Pravda, which McGrath's clumsy satire fleetingly resembles, David Hare and Howard Brenton had the sense not to underestimate their villain.

As the tartan figure of Grandpa Jock, Sylvester McCoy whips up the audience so that we all have to participate in giving birth to Scotland - winningly played and sung by Maria Miller - who has been in the womb since 1707. If an independent Scotland would bring an end to shows like this at the Festival, my vote's for the SNP.

Theatre details: Going out, page 14.