Waiting for Godot, Theatre Royal Haymarket, LondonTime and the Conways, NT Lyttelton, London Peer Gynt, Barbican, London

Ian McKellen and Patrick Stewart are on superb, unstarry form as they breathe fresh life into Beckett's tragicomedy

Time well spent: that sure as hell isn't how the passing years feel to Vladimir and Estragon, Samuel Beckett's tragicomic tramps in Waiting for Godot. For them, time is an endurance test as they kick their heels in a desolate wasteland, going nowhere on a kind of eternal loop-tape.

Indeed, each wretched minute strikes them as so interminable that they keenly debate what hope they have of hanging themselves successfully from the nearest tree. They'll babble about anything, like joke-philosophers, just to keep the frightening sense of emptiness at bay.

With Ian McKellen and Patrick Stewart on superb unstarry form as the ragged down-and-outs, what's brilliant about Sean Mathias's new production is how it fuses the duo's existential anxiety attacks with really delightful warm humour. How fresh they make this 1950s modern classic seem, too. Almost new-minted.

It's all in the delivery. Rather than pay obvious homage to Beckett's legendary poetic minimalism, Stewart and McKellen (using his native Yorkshire accent) sound startlingly natural. What Beckett captured, they reveal, was the rhythm of rambling, funny conversations between true old friends.

Mathias lets more stylised comedy – from the music hall tradition – slip in gradually, and it's a joy to see two such stellar performers, in their battered bowler hats, launching into nonchalant, shambling little jigs when their characters are feeling perky.

McKellen's Estragon is wonderfully entertaining when he snaffles a carrot, giving it a lightning-speed lick from the tip then munching frenetically with his mouth askew. And in his doddery, senile moments, he's like a retarded little boy, tottering on bruised feet to rest his chin on Stewart's shoulder, with a vacant stare.

Stewart's Vladimir is subtly differentiated. He has the air of a lapsed academic, is mentally sharper and morally kinder. Poignantly, he adopts the role of a fond older sibling, smiling and determinedly stoical. Yet he's the more fearful underneath: horrified when he finds himself fleetingly alone, perhaps completely forgotten, as if he had never existed.

By comparison, Simon Callow is a bit of a bore as the bombastic Pozzo. And Ronald Pickup as Lucky, his abused lackey, could be more distressingly downtrodden. Still, I could have watched McKellen and Stewart forever. In the great scheme of theatre-going, Godot is an evening extremely well spent. A splendid start for Mathias's new regime as artistic director of the Haymarket.

In Time and the Conways, over at the National, the hands of the clock spin forward then back again. JB Priestley's drama begins just after the First World War, in the titular family's mansion in a provincial town.

Here Francesca Annis swans around as the widowed matriarch welcoming home her favourite son, Mark Dexter's demobbed, swaggering Robin. His adoring sisters rush in and out, performing charades for their guests. This is the 21st birthday of Hattie Morahan's sensitive Kay – a would-be novelist – and everyone is dreaming of a bright future. Jump to Kay's 40th year, though, and hopes have been dashed. The property must be sold, romances have soured, careers atrophied. Kay has dwindled into a journalist. Yes, it's bleak.

What's worse, though, is having to sit through this sorely disappointing revival. Priestley's middle name was surely "B-rate". This is, essentially, a rehash of Chekhov with a thesis inserted on how all time, including the past, may be eternally present. It's a comforting idea but theatrically clunking, taking the form of a mini-lecture in Act Two.

Alas, some of the blame lies with director Rupert Goold, making a below-par NT debut. Some of his cast are excellent: Paul Ready as Kay's painfully shy brother, Alan, and Adrian Scarborough as the mocked businessman, Ernest, who turns chillingly nasty. Nevertheless, the social satire produces uneven caricatures and Annis turns dismayingly hammy. Goold inserts some hi-tech coups de théâtre, with a frozen moment dazzlingly caught on a spinning set, and multilayered projections of Kay and Alan – past and present – like eerie cubist ghosts. But elsewhere the blocking is peculiarly old-fashioned.

Lastly, there's Ibsen's sprawling episodic saga, Peer Gynt. I confess I glanced at my watch once or twice in the course of Colin Teevan's feisty, expletive-strewn modern adaptation. But Dominic Hill's production (co-presented by the Dundee Rep Ensemble and National Theatre of Scotland) is vibrant and visually stunning.

Hill's vision of the young Peer (Keith Fleming) as a lager-swilling fantasist and borderline mental case – running amok in some small Highlands town – proves remarkably coherent as a concept.

The whole thing is like a substance-abuser's nightmare, slipping in and out of hallucinations, the trolls a gang of druggie underworld sadists, with monstrous ratty tails.

Hill also has a brilliant grasp of pacing, relieving the mayhem with moments of unearthly quiet. Naomi Wilkinson's epic set design is equally inspired: spare, seedy and strangely beautiful. Recommended.

'Waiting for Godot' (0845 481 1870) to 30 Apr; 'Time and the Conways' (020-7452 3000) to 27 Jul; 'Peer Gynt' (0845 120 7550) to 16 May