As "American Idol" heads into its series finale on Thursday, UPI has made a list of the top five most memorable contestants who have competed on the long-running and iconic reality singing competition.

5. Katharine McPhee (Season 5)

Although she came in at second place during Season 5 of "American Idol," Katharine McPhee has enjoyed a successful television career following her loss to Taylor Hicks, landing starring roles in shows such as "Smash" and "Scorpion."

McPhee released her self-titled debut album in 2007 with the hit single, "Over It."

4. Adam Lambert (Season 8)

Another "Idol" runner-up, Adam Lambert has enjoyed a top-selling musical career following his loss to Season 8 winner Kris Allen. Lambert made music history in 2012 when he became the first openly gay male artist to top the Billboard 200 with his second album "Trespassing," which debuted at number one.

Altogether, the 34-year-old has sold 2.5 million albums and 5 million singles. Lambert is currently on tour with legendary rock band Queen.

3. Jennifer Hudson (Season 3)

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Again proving that losing "American Idol" can still lead to success, Jennifer Hudson, who placed all the way down in seventh place on Season 3, has become a household name. Hudson has sold 1.4 million copies across her three albums "Jennifer Hudson," "I Remember Me" and "JHUD," and became the only "Idol" contestant to win an Academy Award for her role as Effie White in the 2006 musical drama, "Dreamgirls."

Hudson was recently in a Broadway production of "The Color Purple."

2. Carrie Underwood (Season 4)

One of country music's biggest stars, Carrie Underwood rose to the top after releasing her debut album "Some Hearts" in 2005, the same year she was crowned the winner of Season 4. "Some Hearts" would go on to be named the fastest selling debut country album in history and the best-selling solo female debut album in country music history. The most successful "Idol" alum, Underwood has sold more than 15. 9 million copies in her career, more than any other contestant.

Most recently, Underwood released her fifth studio album "Storyteller" in 2015.

1. Kelly Clarkson (Season 1)

The original "American Idol" champion, Kelly Clarkson's Season 1 victory and ensuing success helped launch the "Idol" brand and pop culture's obsession with reality-based music competition shows. Clarkson used her groundbreaking victory over runner-up Justin Guarini, whom she starred opposite in the failed 2003 musical comedy "From Justin to Kelly," to launch a lucrative music career that has spanned 13 years. Selling more than 13.9 million records across seven albums, Clarkson truly represents what it means to become an American Idol.

Last releasing "Piece by Piece" in 2015, Clarkson, along with Underwood, will perform during "Idol's" Season 15 finale.