Sentence:  The Leningrad Kirov Ballet , which opened a series of performances Friday night at the Opera House , <WSD>is</WSD> , I think , the finest `` classical '' ballet company I have ever seen , and the production of the Petipa-Tschaikowsky `` Sleeping Beauty '' with which it began the series is incomparably the finest I have ever had the pleasure of witnessing . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)