1. Sarah Paulson
A.K.A. young Ellis Grey.
Before she had Alzheimer's, Meredith's mother was the star of Seattle Grace hospital. Not only did she help with multiple cases, but we also get a glimpse into her relationship/affair with Richard Webber.
2. Christina Ricci
Ricci played Paramedic Hannah, who opted out of dying by running away from the bomb in the body cavity.. only to have herself replaced by Meredith Grey. Hannah is then treated by George, and she learns to accept that it's okay to be afraid and to not want to die.
3. Kyle Chandler
Also in the same episode as Ricci, Chandler plays somewhat-douchy bomb squad leader Dylan. And while we sort of dislike him because he's pushy and a self-described "ass," he's super hunky so we'll forgive him.
4. Seth Green
Green plays patient Nick Hanscrom, but he's better known as the guy with a tumor in his neck. Little Grey is charged with looking out for him, and while making him laugh, his artery/tumor explodes and it gets a little messy in his room.
5. Demi Lovato
Demi plays suspected schizophrenic Hayley, but it turns out that she actually has some weird hearing issue. Her favorite line was "I'm not crazy!"
6. Mandy Moore
Everyone loved when Mandy Moore came on to play Mary Portman on "Grey's." However, her episodes were always wrought with devastation and sadness - first Gary Clark, the hospital gunman, came in to SGMW to shoot up the place, and later, in the mini docu-series that "Grey's" imitated, it's revealed that she dies.
7. Monica Keena
Keena played Bonnie Crasnoff, the woman with a metal pole piercing her abdomen and attaching her to another patient. Bonnie's role was crucial to the Mer/Der/Addison storyline, because when the doctors stopped trying to save Bonnie's life, Mer was left yelling at everyone in the OR, screaming "we can't just forget about her!"
8. Sharon Lawrence
We always knew Izzie had a weird relationship with her mother, but Sharon's appearance on the show was always great. Too bad she can't ever come back...
9. Mae Whitman
Whitman played Heather Douglass, the girl with some rare genetic disorder that made it impossible for her to stand straight up, so she was always facing the ground. Mae was hot-headed and sassy, but after her corrective surgery, she was super happy and thankful to everyone.
10. Abigail Breslin
Breslin played Megan Clover, a nine year old who was weirdly tolerable to literally any injury that came upon her. Turns out, she just couldn't really feel anything except extreme pain, but we still think she's a super hero!
11. Keke Palmer
Palmer played pregnant teen Cheryl Jefferies who was afraid of having a child.
Her performance was great, but mostly, we know that Palmer can literally do anything and everything perfect that she sets her mind to.
12. Vanessa Marano
Marano was kidnapped Holly Wheeler. It was an interesting storyline, but Wheeler did a great job at playing the part!
13. Danielle Panabaker
Panabaker played Kelsey Simmons, who accepts her boyfriend's proposal after only four dates because, like he says, "they've already gone through so much together." Sweet :)
14. Liza Weil
She can play so many different roles - we've learned to love her on "Gilmore Girls" and "HTGAWM," but we really loved her softer side coming out when she played Alison Clark on "Grey's" - becoming best friends with cancer-stricken Izzie was one of our favorite moments!
15. Jurnee Smollett Bell
THIS GIRL BROUGHT DEREK SHEPHERD AND MEREDITH GREY BACK TOGETHER.
16. Kevin Rahm
Mr. Duff, the psychic. Not much to be said here except that he was weird and it was awesome.
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AND comment below to add any of your favorite guest stars on "Grey's Anatomy"!