When it comes to 20th-century string music the Arditti String Quartet is the very best. In fact, history will probably prove the British foursome to be one of the finest ensembles in any category, playing music of any period. What makes the Arditti so nearly peerless is the fact that they apply to the contemporary repertoire the same exacting standards expected from chamber ensembles playing music from the standard classical canon. Violinists Irvine Arditti and Graene Jennings, violist Garth Knox, and cellist Rohan de Saran combine precision, clarity, and deep respect for the composer's work (no matter how idiosyncratic or unconventional) with passion, intensity, and, where appropriate, expressivity. The Arditti's growing discography, particularly on the French Disques Montaigne label, contains definitive interpretations of much of the corpus of contemporary composition, from the revolutionary Viennese composers Arnold Schoenberg, Anton Webern, and Alban Berg to some of the most innovative but less widely known of today's composers, like Younghi Pagh-Paan, Jay Alan Yim, Georges Aperghis, Helmut Lachenmann, and Henri Dutilleux. On the program at this concert are two Viennese classics: Webern's early Five Pieces, from 1909, and Berg's stunning Lyric Suite, composed in the mid-20s. The quartet will also perform French composer Dutilleux's 1976 piece Ainsi la nuit, which has a sensual lyricism and nuanced gentility, as well as very deep harmonic and structural implications. Perhaps the most eagerly anticipated item is American composer Charles Wuorinen's Piano Quintet, a world premiere featuring the brilliant pianist Ursula Oppens, who's currently teaching at Northwestern. Sunday, 3 PM, Athenaeum Theatre, 2936 N. Southport; 722-5463 or 663-1628.