From most restrictive jurisdictions to least:

If you're in a hardcore jurisdiction there's completely clandestine products you can build like FGC9 (a 9mm short carbine made with 3D printing and chemical processes)

Depending on where you are in the world you might be able to get a kit with near-finished components (called 80% in USA) and jigs to complete them yourself.

You may have access to lethal weapons that are not technically firearms, or are not regarded as ordinary firearms. For example, there are some high-powered air rifles that may be adequate for self-defense. Another category that is often less regulated is black-powder (historical) firearms, some variants of which use or can be converted to use old-style cartridges ("cowboy loads").

Assuming you can buy a proper firearm at all, if you're just looking for home defense you can get "suboptimal" weapons which comply with the restrictions in your area, such as shotguns or lever guns. These will be quite sufficient for that purpose. This is likely to be the case in commie states of the USA; in UK I think break-action shotguns are supposed to be more readily available than a proper modern gun as they are more often used for sport.

Finally if you have basically unrestricted access you want the dominant battle rifle of your region (either AR15/M16 type or AK47 type), a concealed carry pistol/revolver, and possibly a pump or semiauto shotgun (eg in USA Mossberg Maverick 88).

Some advice for concealed carry: 32ACP (ball FMJ), 38 Special (wadcutter) and 380 ACP get plenty of penetration, so don't be fooled into think you absolutely must get 9mm or larger (which most gun people will tell you based on cops & military using 9mm+, but officers engage over distance, through cover, and are fine with bleeding out to eventually stop a target). These will generally be smaller which makes them more convenient to carry but harder to shoot well.