Does 'iconic voice' means easily recognizable and often imitated?
If so, I personally would't call Frank Sinatra's voice iconic, I always confuse him with Dean Martin and Andy Williams (to a lesser degree) and a few other male singers of his time. Don't get me wrong, he's a legend, but his voice is very regular.
Britney's voice has been more ridiculed than praised don't be petty, no one is a music snob, you were more than welcomed to voice your opinion but you should have been prepared to have someone who would disagree with you for obvious reasons
Can you post a single publication or piece of acclaim supporting your claim? Because I can list more than a dozen from any person on my list.
Britney the personality is iconic, her voice isn't. Just as Marilyn Monroe as a persona is iconic yet her acting isn't. So because anything relating to an icon is iconic, is Britney's pooch iconic too?
The nasal voice is definitely iconic But if you don't like the word iconic, Britney's voice is definitely famous. The amount of celebrities alone who cover her songs by adding a nasal element to their voices is enough to cement her voice as one of the most famous of all time.
James Brown, Luciano Pavarotti, Frank Sinatra, Barry White, Muddy Waters, Louis Armstrong, Janis Joplin, Bob Dylan, Johnny Cash, Aretha Franklin, Freddie Mercury, Amy Winehouse, David Bowie, Whitney Houston, Mariah Carey.
Britney, Christina and Shakira as far as this generation of pop is concerned. No where near the level of the above though.
Every generation has their group of singers that are constantly brought up as someone's influence. For this generation it's: Aretha Franklin, Mariah Carey, Whitney Houston, Janet Jackson, Aaliyah, Xtina, Amy Winehouse.....I see many people claiming Rihanna as an influence 5-10 years from now.
Other than her accent, Bey's voice never really had a strong identity as her peers then and now. While she is hands down the best vocalist in this generation of pop music. She always reminds me more of other people than she herself.
But if you don't like the word iconic, Britney's voice is definitely famous. The amount of celebrities alone who cover her songs by adding a nasal element to their voices is enough to cement her voice as one of the most famous of all time. 
I love her but come on now.