Shotguns. Mossberg 500 or Remington 870. Why let the cops impound your nice Beretta or Benelli after a home defense shoot? FWIW, guns that look like they are used for duck hunting probably go over better in court than evil black guns with pistol grips. Just food for thought.
If you are made of money, a Benelli supervinci auto would be nice
Anything pump or auto. You don't want to get an over-under or side by side with only 2 shells in it.
Handguns, really way too numerous to list.
Some kind of .357 magnum revolver like this S&W K-frame. You can always use .38 special or +P.
Or Colt Python
Or any full size hand gun in 9mm, .45 ACP or .40 S&W. There is just so many of them. Here are some classics.
Beretta 92fs, or if you have no money, Taurus makes a clone of this.
Walther P99
Kimber 1911
Sig P220 or P226
Browning Hi Power
H&K P7M8 or M13
Glock 17 or 21
H&K USP
The choices for great hardware are mind boggling. Try to rent or borrow and just see what you like.
Mossberg 500 in 20 gauge is a fine weapon, I gave one to Mother Slim back in the 80’s. Really, there’s no real difference in 12 vs. 20 gauge ballistics, just the weight of the projectile(s). Most 12’s throw 1 1/8 ounce at 1200-1300 feet per second, versus the 7/8 to 1 ounce in a 20. The 20 will be a smaller frame gun and easier to handle.
For a handgun I’d suggest a Smith & Wesson .357 in a K frame or L frame 4” barrel, and shoot it with .38 Special +P rounds. A variety of grips are available so it will fit just about anyone’s hands. The Pachmayr Compact is a good place to start.