For me, I find the radial menu to be the simplest and most comfortable to use, and fits in line with other shooter games like the modern Doom games. One addition I would make is that if you select an already held weapon with the the opposite mouse button (e.g. right-clicking the weapon held on your left hand), that both weapons swap hands.
The back slot seems ok as they don't require usage of the mouse (as it's already used for aiming), but is limited by its control scheme only allowing for one weapon to be in the backslot, compared to the radial wheel and even the UI buttons can be expanded to have many weapon slots.
The throw box is interesting but it's wildly different from the other options that allow for more freedom in terms of weapon selection, so it may end up impacting the core design of the game and how players interact with weapons.
I personally am not a fan of the UI boxes, as they're positioned in the corner which diverts the attention away from the player character and the game as a whole when switching weapons. The radial wheel is better in this regard as the wheel appears on the player themselves.
