Answer by taking a quote from the following article:

Diamond Rio is an American country and Christian country music band. The band was founded in 1982 as an attraction for the Opryland USA theme park in Nashville, Tennessee, and was originally known as the Grizzly River Boys, then the Tennessee River Boys. It was founded by Matt Davenport, Danny Gregg, and Ty Herndon, the last of whom became a solo artist in the mid-1990s. After undergoing several membership changes in its initial years, the band has consisted of the same six members since 1989: Marty Roe (lead vocals, rhythm guitar), Gene Johnson (mandolin, guitar, fiddle, tenor vocals), Jimmy Olander (lead guitar, Dobro, banjo), Brian Prout (drums), Dan Truman (keyboards, organ, synthesizer), and Dana Williams (bass guitar, baritone vocals).

After Olander, Williams, and Johnson had recovered, the six musicians set to work on their debut album. In doing so, Johnson soon discovered that the injuries to his hands had altered his dexterity on the mandolin, and threatened to walk away after Powell offhandedly remarked that he would have Roe dub in his own tenor harmonies instead of having Johnson sing them. The band also had commitments to finish as the Tennessee River Boys, to the point that they occasionally had to promote themselves under both names in the same day. Arista Nashville released Diamond Rio's debut single, "Meet in the Middle", on February 6, 1991. As the lead single to their self-titled debut album, "Meet in the Middle" went on to spend two weeks at No. 1 on the Billboard Hot Country Songs charts, making Diamond Rio the first country music group ever to send its debut single to the top of that chart. Following its release, the band performed its first official concert as Diamond Rio on May 4, 1991. They shared the bill with Wild Rose, whose membership included Prout's then-wife, Nancy Given Prout. Released three weeks later with DuBois and Powell as producers, Diamond Rio was met with positive critical reception from critics such as Allmusic, Chicago Tribune, and Entertainment Weekly, which praised the band's vocal harmonies, instrumentation, and song choices. Four more singles were released from Diamond Rio, all reaching top 10 on the Billboard country singles charts: "Mirror, Mirror", "Mama Don't Forget to Pray for Me", "Norma Jean Riley" (which was previously the B-side of "Mama Don't Forget to Pray for Me"), and "Nowhere Bound", the latter two of which were co-written by Powell. Roe and Prout had found both "Mama Don't Forget to Pray for Me" and "Mirror, Mirror" by attending shows at the Bluebird Cafe in Nashville, a popular spot for performances by aspiring songwriters. Truman and an employee of Arista had found "Norma Jean Riley", which was originally titled "Pretty Little Lady" until DuBois remarked that the lady in the song should have a name: "It could be 'Norma Jean Riley', anything!" Johnson spoke positively about "Mama Don't Forget to Pray for Me", which was written and originally recorded by Larry Cordle, and the impact that it had on fans. He recalled a letter sent to him by a female fan who had run away from home and chose to return after hearing that song, and said that "We already didn't wanna do the drinkin' songs and stuff ... if you're gonna touch someone, touch them with something that's positive." Diamond Rio was later certified platinum by the Recording Industry Association of America (RIAA) for shipping one million copies in the United States. In addition, the band won the Academy of Country Music's Top Vocal Group for 1992, an award they would receive again in 1993, 1994, and 1997. They were also nominated for Top New Vocal Duet or Group by the same association in 1992. A cut from the album, the instrumental "Poultry Promenade", gave the band its first Grammy Award nomination. CANNOTANSWER

What was their best single in 91-92?

A cut from the album, the instrumental "Poultry Promenade", gave the band its first Grammy Award nomination.