The Knicks have turned the Big Apple into the Big Party. The NBA champs sent thousands into the streets after knocking off the San Antonio Spurs on Saturday night, with everyone from MTA bus drivers to city firefighters joining the frenzy — while massive crowds sang along to Sinatra’s “New York, New York” in one monster celebration. And that party’s just starting . “I think it’s unmatchable,” West Village actor Katya Ferrer told The Post Sunday. “I saw someone today say it’s like a reverse 9/11.