Hers is the finest voice in musical theater today though critics are still divided over Betty Buckley's performance in Broadway's Sunset Boulevard. But on this everyone agrees: Betty can sing a sentiment justified by the glorious live record The London Concert, populated mostly by Broadway standards, including ''Memory'' and ''Meadowlark.'' Some selections are a bit predict able, but her renditions are definitive, moving, and letter-perfect. And now that Buckley's taken her turn, we can impose a moratorium on ''Send In the Clowns.'' A

