Denise Richards' Daughter Posts Loving Tribute to Patrick Muldoon

Patrick Muldoon and Denise Richards broke up more than 25 years ago ... but he played a huge role in her family life -- so says her daughter Lola Sheen.

Richards' middle child shared a throwback photo of the "Melrose Place" star standing alongside her mom ... writing, "the best duo there was" on the pic.

She added a long caption to the post which begins, "i thought you were the one that was supposed to be in our lives forever" ... adding it's going to be weird to move on without him given how important he was to her childhood.

Lola shared a fun memory of Patrick repeatedly singing "that Lola song" to her -- unclear what she's referring to, though Barry Manilow's "Copacabana (At the Copa)" starts off with a famous reference to a Lola -- which made her laugh super hard.

She finishes it off by saying she repeatedly begged her mom to get back together with Muldoon ... but notes "staying friends was better" anyway -- wishing him the best in the afterlife.

Muldoon and Richards met while shooting "Starship Troopers" in 1997 ... striking up a romance which lasted until about 2000. The two remained close friends for years -- though Richards has said she put some distance between them during her marriage to Charlie Sheen, Lola's dad.

After she and Sheen split, Muldoon came back into her life, and the two shared a special relationship until he died. The two starred in the 2015 film "A Christmas Reunion" together ... and he was a guest at her and Aaron Phypers' 2018 wedding. He also appeared on "Denise Richards & Her Wild Things" last year.

As we reported ... Patrick passed away in his Beverly Hills home Sunday after suffering a heart attack. He was found unconscious by his girlfriend on the bathroom floor. Paramedics arrived at the house but were unable to revive him.

He was 57.