You have a standard 12 gauge shotgun, maximum chamber 3". You can only have one round with you in a survival or self defense scenario, what would it be?

Winchester Super-X Ammunition Buffered # 1 Buckshot, 12 ga., 2 3/4"

This year for us that load of choice may change.  In general a conversation like this is a bit counter to what a Shotgun really is about but I get your question.

We are going to test the Winchester segmented slug in my wife's 20 gauge M2.  We keep the shotguns loaded with slugs due to larger critters but this Winchester slug has some interesting features.  Basically if it hits soft material like cloth or flesh it will segment in to 3 pieces, acting like buck inside the body.  If it hits hard material first like wood, the hollow point will fill up and the slug won't segment. So now it acts like a solid slug and punches holes for deeper penetration.  If this works we will put away a fair amount of it for sure.

It really depends on what you intend to shoot and where, this would work for us.

Whichever 2 3/4 inch slug works best in your shotgun.

In my home, I load Federal #1 Buck. If we're bring the outdoors into account, I suppose I'd go with Federal Flite Control 00 Buck, if I could only pick one.

I would take a good buffered #6 shot. I would use that to make a few slugs

Not much beats good. old 00-buck. There is an amazing variety of that load out there today,
but it's hard to beat a handful of .33 pellets simultaneously slamming into a target.

3" mag T shot.   It's my go to on mid to long range geese, and I wouldn't want to get shot by it.  I'm pretty sure it's about the same size dia. As #4 buck.

Any good hollow point slug. It's a massive chunk of lead (1oz or more in 12g) and it can kill pretty much anything that moves at up to 300m. Try that with shot.

I'm  with you here 53 .20 cal. Steel balls should do for just about anything. If I could find F shot I might go with that.

4 buck.  Not ideal for anything but could be used on some smaller game and for defense. I would never limit myself that way though.  6 shot, 1 buck and DP slugs.

I guess a 3 in. 1 1/4 oz. slug.

3"mag #4 buck, buffered.

The Remington load has 41 pellets, the most I have found. That would be my choice as well. I used to shoot varmets with it. Smaller game can walk through the pattern of 00 buck, slugs defete the purpose of a shotgun, birdshot is for the birds. number 4 buck (not to be confused with number 4 birdshot) is good enough for CQB, small to medium game. Probably not suitable for deer sized game at hunting distances.