Mr Red, I completely agree that not all lyrics should emphasize vowel sounds. But the original question was about which sound to hold, and in general, even consonants that can be held (m, n, s, z, f, l, r, for example) don't sound that good when you do hold them. (Some exceptions noted above.) 'Punching' the consonant, e.g., using a very distinct "t" at the end of a word, can be very effective in some songs.
One song that comes to mind that makes much merriment out of repeating and emphasizing a consonant is "Mary Mack."
"Mary Mack's mother's makin' Mary Mack marry me,
My mother's makin' me marry Mary Mack..."


BTW, it's worth mentioning that when using a microphone you've got to be careful about consonants such as s, t, p, k, and b.

Genie