First would be Straight no Chaser
Men's Jazz Chorus group. Let me just say....wow. The blend of their voices is so tight all of them are just so into what they do. It makes the songs just pop out. Mom-in-law just got me and the wife the Christmas album that they just came out with.
Second is Celtic Woman
Woman's group singing (most of the time) in the Celtic style. They have such rich and pure voices. Their vowels match perfectly and the harmonies give me goosebumps. This WILL be my next music purchase.
And it doesn't hurt that they're easy on the eyes.
Throw out some other groups that y'all like!