Whatever you learn will help, so long as you have a good instructor, and get a chance to see how the style works outside of the studio or ring. All the arts mentioned here have useful, practical aspects that, together, can make someone a more complete combatant.
Consider ground fighting, striking, learning to get struck, weapons, clinch/stand-up grappling range, multiple opponents, and then adding in the dynamic of a firearm (yours or theirs). Don't neglect fitness as an underlying necessity to all of this. Depending on your strengths, weaknesses, and likely encounters, there really isn't a bad choice, except to not seek training.
There are a few good martial arts threads on here, though they're a bit old. Fortunately, none of them devolved into a "whose kung fu is best" pissing match.
The advice above is worth exactly what you paid for it.
Bookmarks