Nashville Bluegrass Band and Blue Highway are both terrific bands, have many good albums. Ralph Stanley's "Saturday Night and Sunday Morning" is a wonderful collection with many other singers joining him. Any of the Bluegrass Album Band's albums are good. There is no one best bluegrass album in my opinion but many very good tracks on many very good albums. Your best bet is to get a few samplers with tracks from various bands.

On the other hand, if you're coming from a Celtic background, you might want to get something by Tim O'Brien, who is a great singer, songwriter and musician in both Celtic and bluegrass genres, along with anything else he decides to play. He was in the former band Hot Rize, but has also put out albums with various other of his own bands.