Neither album is the best work by the respective band, but I voted Reign in Blood because it sucks less than Master of Puppets does, when compared to each band's better work. Does that make sense?
I just bought my brother-in-law a copy of this CD for Christmas last year, and he's not a metal fan at all. He asked me what the essential metal CD to own was and you know the drill...
I also told him to pick up Minor Threat discography CD.