Liver shot. I train in boxing, and I'm a southpaw, so the way that I stand when facing a right-handed fighter (my right shoulder is in front of me, their left shoulder is in front of them) and vice-versa means that their liver is close to my fist (and vice-versa). So I can hit right-handed opponents in the liver with a well-placed low right hook. In a right-handed vs right-handed scenario, you'd have to throw a looping punch with your rear hand to hit their liver (much harder to do effectively than a hook punch from your lead hand)
90% of people are right-handed, which works very much in my favour. The liver shot is my go-to move. I like to duck under my opponent's jabs, and then counter with a liver shot. Only problem is, I'm not fast enough to do it efficiently, so I often end up eating a right cross at the same time that the liver shot connects.