If I had a scale, it would look something like this:
1) Great
2) Very Good
3) Good
4) Pretty Good
5) Decent
6) Passable
7) Mediocre/Average
8) Below Average/Subpar
9) Bad
10) Very Bad
11) Extremely Bad
12) Terrible
13) Horrendous
14) Bob Dylan Bad
However, before your head starts swelling up any bigger, I should make a few clarifications:
1) Great
On a purely vocal level, there are very few Great singers. A great singer requires a large vocal range, the ability to sustain notes, the ability to project, the ability to express emotions, while maintaining a distinctive sound, style, and personality.
If I played someone a record by Elvis Presley, Nat "King" Cole, Frankie Laine, Frank Sinatra, Johnnie Ray, Doris Day, Johnny Mathis, Bill Kenny, Mario Lanza, Jane Monheit, Crystal Gayle, Pat Boone, Kay Starr, Patsy Cline, Georgia Gibbs, Judy Garland, Bing Crosby, Thurl Ravenscroft, Eddie Fisher, Tiny Tim, Vaughn Monroe, Sarah Vaughan, Al Jolson, Tony Bennett, Jim Reeves, Teresa Brewer, Shirley Bassey, Connie Francis, Dean Martin, Dion and other greats, they would recognize them -- even if they'd never heard the song before. That's what makes them great.
2) Very Good
Most *successful* singers fall under the Very Good category. These singers have flawless voices and enough vocal personality that one can sometimes recognize them; or they have recognizable, though somewhat limited or flawed voices. Dinah Shore, Patti Page, Guy Mitchell, Vic Damone, Perry Como, Andy Williams, Ella Fitzgerald, Billie Holiday, Gogi Grant, Kate Smith, Joni James, Jane Morgan, Jaye P. Morgan, Anita Bryant, Leslie Uggams, Jimmy Durante, Louis Armstrong, Sammy Davis Jr. and many others.
Note: These are all truly wonderful singers, and among my favorites. They simply fall slightly short (for one or more reasons) of the Greats.
3) Good
Good singers are fairly successful and highly talented, but lack definable vocal characteristics in terms of vocal personality and style. Rosemary Clooney, Johnny Desmond, Evelyn Knight, Margaret Whiting, June Valli, Eydie Gorme, Alan Dale, Jerry Vale, etc.
4) Pretty Good
Pretty Good singers don't quite cut it. They might have some great records, but it's not due more to the song and the arrangement than to their vocal talents. Pretty good singers didn't last long where the standards are concerned. Mike Douglas and Merv Griffin were pretty good big band vocalists, but they're better remembered for their work as talk show hosts. The majority of r'n'r singers, however, fall into this category. Again, this is not to diminish their accomplishments and/or talents in any way, but is simply an acknowledgement that the r'n'r form demands less from the vocalist: Bill Haley, Buddy Holly, Jerry Lee Lewis, Fats Domino, Chuck Berry, et al., were all pretty good singers. The same holds for the lead singers of most successful later groups: Jim Morrison, John Fogerty, Eric Burdon, Roger Waters, et al.
5) Decent
In the world of standards, there are no Decent singers. They wouldn't have gotten a record deal, or a spot with a big band or a radio program.
In the rock/post-rock era, they are legion. Decent means that while I won't change the channel because of the singer's voice, I'm not listening because of it, either.
Son Bono was a decent singer. Cher is very good.
6) Passable/Mediocre/Average
A passable singer could sing at an open mic without getting booed off the stage. An average singer can sing in key, carry a tune, and hold their end notes for an acceptable amount. However, there is nothing even remotely unique or memorable about their voice/style/approach.
The remaining categories represent successive degrees of badness:
7) Below Average/Subpar
As the header says, below average. I couldn't sit through an entire song by them. They may sing a little sharp or flat, or hit an occasional sour note, have a weak delivery, etc. Think Nelly Black.
8) Bad
I'd leave after the first 5 seconds. Michael Jackson.
That's a joke. MJ can sing in key. Sandy the Screech Owl Madaris is bad.
9) Very Bad
I'd stick my fingers in my ears at the first note.
Name the lead singer of virtually any metal band or rap group. I find the growling, grunting, and barking to be extremely displeasing, but I don't experience any actual physical pain from it.
10) Extremely Bad
I'd stick my fingers in my ears at the first note and run. The Bee Gees, Andy Gibb, Shaun Cassidy, disco in general.
Falsetto (disco style) is like nails on a chalkboard to me.
12) Horrendous
I'd scream, stick my fingers in my ears at the first note, run, and be emotionally scarred for life. Janis Joplin and Joe Cocker, for example.
13) Bob Dylan Bad
I'd rather ram knitting needles into my eardrums than listen to his whiny-assed voice.