Question: Who did background vocals and co-wrote "Best Days of Your Life"?
Context: Restless Heart is an American country music band established in 1984.  The band's longest-lasting lineup has consisted of Larry Stewart (lead vocals), John Dittrich (drums, background vocals), Paul Gregg (bass guitar, background vocals), Dave Innis (piano, keyboards, rhythm guitar, background vocals), and Greg Jennings (lead guitar, mandolin, background vocals).  Verlon Thompson was the band's original lead singer, but he was replaced by Stewart before the band signed to RCA Records Nashville.  Stewart departed for a solo career in 1992; during this time, Innis, Gregg, and Dittrich alternated as lead vocalists, until Innis also left.  The band was on hiatus from 1994 to 1998.  During this hiatus, Dittrich briefly recorded as one-third of The Buffalo Club, and Jennings joined Vince Gill's road band.  Stewart, Gregg, Dittrich, and Jennings reunited briefly in 1998 to record new material for a greatest hits album; they, along with Innis, reunited again in 2004, and this lineup has remained ever since. Lonestar is an American country music group consisting of Richie McDonald (lead vocals, acoustic guitar), Michael Britt (lead guitar, background vocals), Dean Sams (keyboards, background vocals) and Keech Rainwater (drums, percussion).  Before the group's foundation in 1992, both Rainwater and Britt were members of the group Canyon.  John Rich (bass guitar, lead and background vocals) was a member until he left in 1998, and later became one half of the duo Big & Rich, as well as a Nashville songwriter and record producer.  Between 2007 and 2011, McDonald exited the band for a solo career, with former McAlyster lead singer Cody Collins replacing him until McDonald rejoined. Williams Riley was an American country music band composed of Steve Williams (lead vocals), Derek George (guitar, background vocals), Charlie Hutto (guitar, background vocals), Joe Rogers (steel guitar, fiddle, harmonica), Dave Guidry (bass guitar), Travis Thibodaux (keyboards, background vocals) and Nick Buda (drums).  Derek George had formerly played guitar for Bryan White, and before that, he was a member of the band Pearl River.  Williams Riley self-released its debut album in 2009 and has charted on the U.S. country charts with "Country Livin'."  This album includes collaborations with White, as well as Slash and Edwin McCain.  The band's self-titled debut album received a favorable review from Matt Bjorke of Roughstock, who called it a "well-crafted album of contemporary country music." The Nobility is a rock and roll band based in Nashville, Tennessee, currently composed of Sean Williams (lead vocals, rhythm guitar), Stephen Jerkins (background vocals, keys), Cheyenne Medders (background vocals, bass) and Will Medders (background vocals, drums).  At certain performances they are joined by a third Medders brother, Carson on lead guitar. This Time It's for Real was the second album by seminal New Jersey Rock/R&B band Southside Johnny and the Asbury Jukes.  Like their first album "I Don't Want To Go Home" this album features a number of guest artists and duets, a trend that would be dropped for their next album "Hearts of Stone".  "Check Mr. Popeye" features Kenny 'Popeye' Pentifallo on vocals with The Coasters on background vocals.  The track "First Night" features The Satins on background vocals and Steven Van Zandt on duet vocals.  "Little Girl So Fine" features background vocals by The Drifters.  The album also features a cover of Aretha Franklin's "Without Love". Kellie Pickler is the self-titled second studio album by American country artist Kellie Pickler.  The lead-off single, "Don't You Know You're Beautiful", was debuted at the 43rd Academy Of Country Music awards and peaked at 21 on Hot Country Songs.  The album was released via BNA Records/19 Recordings on September 30, 2008.  Since the albums' release, three more singles have charted; "Best Days of Your Life" at number 9 (which was co-wrote with fellow country artist Taylor Swift), "Didn't You Know How Much I Loved You" at number 14 (a re-recording of an album cut from Pickler's debut album "Small Town Girl") and "Makin' Me Fall in Love Again" at number 30. Atomic Tom is a rock band from Brooklyn, New York, consisting of Luke White (lead vocals), Eric Espiritu (guitar, background vocals), Ethan Mentzer (bass, background vocals), and Tobias Smith (drums, background vocals).  The band have released two albums, two EPs and a number of singles. Little Texas is an American country music band founded in Nashville, Tennessee, in 1988 by Tim Rushlow (lead vocals, rhythm guitar), Brady Seals (lead and background vocals, guitars, keyboards), Del Gray (drums), Porter Howell (lead guitar, background vocals), Dwayne O'Brien (rhythm guitar, lead and background vocals), and Duane Propes (bass guitar, background vocals).  Signed to Warner Bros.  Records in 1991, Little Texas released its debut album "First Time for Everything" that year.  The album's lead off single, "Some Guys Have All the Love", reached a peak of No. 8 on the "Billboard" Hot Country Singles & Tracks charts.  Little Texas continued to produce hit singles throughout the mid-1990s, including the Number One single "My Love" and six more top ten hits.  Their debut album earned a gold certification from the Recording Industry Association of America (RIAA), while 1993's "Big Time" was certified double platinum and 1994's "Kick a Little" was certified platinum. Topmost was a Finnish rock band founded in 1964, becoming the most popular band in Finland in 1967.  Its members were Heimo "Holle" Holopainen (bass guitar, background vocals), Kristian "Kisu" Jernström (drums, background vocals), Vasilij "Gugi" Kokljuschkin (vocals), Eero Lupari (guitar, background vocals), Harri Saksala (vocals, saxophone), Arto "Poku" Tarkkonen (keyboards). "Best Days of Your Life" is a song written and performed by American country artist Kellie Pickler, and co-written by Taylor Swift.  It was released on December 1, 2008 as the second single from her self-titled second album.  Swift also provides background vocals on the song.  The song is about a narrator explaining how her former boyfriend cheated on her and moved on to a new girlfriend to start a family with, wishing them well on their new life but says that the new girl won't top what they previously once had before.
Answer: Taylor Swift