Be cautious with 'Kiss Me, Liar'—it's often recommended in mafia BL lists, but the core relationship is between a boss and a bodyguard/employee, not rival bosses. The power dynamic is heavily skewed with elements of coercion, which is a whole different genre. If you're specifically looking for that equals-clashing-as-rivals energy, this might disappoint you. The story is more about obsession and possession within a hierarchy, lacking that sense of two opposing forces meeting on level ground, even if that ground is a battlefield.
It's a popular and intense title, so it comes up a lot, but it's important to set expectations. The 'rival' aspect is more about external threats to their organization, not an internal rivalry between the two leads. Knowing what you're getting into can save you from a frustrating read if you're craving that specific clash-of-titans dynamic.