For some, it will be the rifle (or the lack there of) they just happen to have in the vehicle. Most people don’t carry a rifle with them in their vehicle so it be whatever rifle they can scavenge.
I think the best rifle with will change over time during a SHTF situation. Depending if you are in the short or long term phase. In the short term, you would still be dealing with societal decay, with a greater threat to human life. So a battle rifle like an AR in 5.56 or 7.62x39 would be my weapon of choice. Something I could use to defend myself and my family, while being able to take medium sized game.
Once in the long term phase of the event (if you are lucky enough to make it that long). It’s likely, some form of stability will have been created. Either through civil agreement or overwhelming force and violence, leaving the dominant group in control. In this phase, while there would still be the threat to human life. Most likely, it would not be as great of a threat one would face in the pervious phase. So a smaller, lighter rifle like a .22lr would be my choice.
With a .22lr, you can carry a large amount of ammo. The large amount of ammo has 2 benefits. 1, more ammo in creases your chances of eating and getting food for the family. 2, having extra .22lr ammo would be good for bartering and trading for other goods while out in the field.
I could be wrong, but I really do think the best weapon systems will evolve over the duration of the event. With factors such as short and long term time phases, what dominant forces are in control, what ammo supplies are available, and what type of game is available for hunting.