Avril:

Best - Let Go/TBDT
Worst - Goodbye Lullaby (though criticism wise I suppose it'd be TBDT)

Blake:

Best - Heartbreak On Vinyl
Worst - Audio Day Dream

Britney:

Best: Britney & In the Zone
Worst : Blackout ( the music was great but she was a mess)

Rihanna

Best: GGGB & Loud (epic music and image, hit after hit)
Worst: MOTS because she only had pon de replay.

Gaga

Best: The Fame & TFM (great music & image)
Worst: Born this way ( bad/underwhelming music, disappointing first single, overhyped and her image)

Taylor Swift
Best- Fearless
Worst- None.

Lady Gaga
Best- The Fame Monster
Worst- pre-2009

Rihanna
Best- Rated R
Worst- Talk That Talk

Selena Gomez
Best- When The Sun Goes Down
Worst- Kiss & Tell

Miley Cyrus
Best- Can't Be Tamed
Worst- Hannah Montana 3-5

Shania Twain
Best- Come On Over
Worst- self-titled debut

I can't @ all of you selecting BTW era as the best.

What era is then?

+1

Avril Lavigne:
Best: Let Go / The Best Damn Thing (Favorite albums from her too)
Worse: Goodbye Lullaby (Most of her songs in this era flopped[With the exception of WTH], and the album was such a disappointment)

GAGA
Best: Born This Way
Worst: Red And Blue

Demi
Best: Here We Go Again
Worst: Don't Forget

Selena
Best: When the Sun Goes Down
Worst: A Year Without Rain

Miley
Best: Can't Be Tamed
Worst: Meet Miley Cyrus

Britney
Best: In the Zone
Worst: Baby One more Time

Rihanna
Best: Good Girl Gone Bad
Worst: Music Of the Sun

If we're talking about the way the era was handled and promo as an era overall, then best is "Britney" and worst is "Blackout." Best WOULD have been ITZ if it wasn't canceled. So Britney is second in line. Musically it'd be different though.