There's a few for me:
Avril Lavigne - Smile
I really liked her on her first two albums and even "The Best Damn Thing" had its moments despite a fast burn rate. And while I liked the lead single "What The Hell" a lot, it was "Smile" that ruined her for me. Way too immature sounding, it made me realize her career was going in the wrong direction, and fast! She has yet to have a big hit for me since.
Nickelback - Rockstar
In the midst of their overplayed 2005-07 period came this song, with their worst lyrical clichés ever. While they did have some decent songs since ("If Today Was Your Last Day" and "Never Gonna Be Alone", in particular), they just haven't been the same since.
Jennifer Lopez f/Fat Joe - Hold You Down
Another case of not totally lost interest, but this really held her down as I felt it was very bland.
U2 - Get On Your Boots
This had a very fast burn rate for me, and to date, they have yet to recover.
Rihanna - Rude Boy
I'd generally found her stuff good before this, but this was truly the first Rihanna single I hated. While she has had some good stuff since (particularly "Only Girl (In The World)" and "S&M"), she hasn't had any real classics for me. Further cementing this status are songs like "You Da One" and "Birthday Cake", easily her worst ever.
Backstreet Boys - Inconsolable
I'd generally liked them up until this point, but this showed a lack of originality, and that their creativity had stalled. Sadly, they haven't had any great stuff since.
Beyoncé - Run The World (Girls)
It's no secret that this is one of my most hated songs ever, and one of the biggest disappointments ever. Despite a few decent songs since, none have come close to reaching the level of her classics for me.
Mariah Carey - Touch My Body
I did initially latch onto the hype for this, but it really died off fast for me... and she hasn't had any really redeeming songs since this for me.
Black Eyed Peas - Boom Boom Pow
For me, this was the beginning of the end for them, as their creativity led to them releasing generic, disposable records. And while the E.N.D. was mediocre for me, The Beginning was even worse, and ever since the horrible "The Time (Dirty Bit)", they've failed to make any kind of impression.
Sheryl Crow - Love Is Free
I'd liked most of her stuff up until this point, but this was really the start of releasing mostly uninspired stuff... sadly, she has yet to recover for me.
Sugababes - Girls
What kind of mess was this? Combined with the drama of their membership changes, this completely made me lose interest in them.
Leona Lewis - Collide
What a disappointment considering how much I'd liked her up until this point... and I doubt she'll recover.
Sarah McLachlan - Loving You Is Easy
We waited 7 years for THIS dull release? I used to really like her, but this was enough to put me off her.
Mandy Moore - I Could Break Your Heart Any Day Of The Week
Sure, one indie-styled album was alright as I still liked her then, but a second was just too much, and this really put me off her.
Dido - Don't Believe In Love
Another long-awaited return that really failed to deliver.
Chantal Kreviazuk - Invincible
Completely failed to register for me. Shame as she's had so many great songs before this.
James Blunt - Stay The Night
Really, all he could do was rip off Train?
Jason Mraz - I'm Yours
That this was my #1 most hated song of all time, especially coming from an artist who I'd enjoyed many songs from before, should say it all.