Kristin Cavallari just took a trip down memory lane -- AKA, revisiting relationships with her exes, and which one she’d get back with ... only hypothetically, of course!

The former "Laguna Beach" star opened up the floor for questions during a live taping of the "Let’s Be Honest" podcast tour in Chicago ... and in this TMZ-obtained clip, one fan asked her which ex she’d spend the rest of her life with, if she were forced to choose.

Kristin wasn’t exactly thrilled with the question, joking she’d "honestly, rather f***ing die," but she was determined to give an answer -- eventually picking her most recent ex, Mark Estes.

KC wasn’t just giving a name -- she had a cheeky reason too, saying she’d be guaranteed the best sex for the rest of her life if she had to get back together with Mark.

She had a few solid options to choose from, including ex-hubby Jay Cutler, her brief fling with Morgan Wallen, and some of her earlier famous flames, like Stephen Colletti and Brody Jenner.

Looks like Mark's earned the ultimate pat on the back -- but even though his bedroom skills were top-notch, it wasn’t enough to make their romance last. Kristin admitted they split because the 24-year-old TikTok star just wasn’t built for the long haul with her.