As everyone who's contributed thus far has most likely also realized - this is a difficult question to answer holistically, without compartmentalizing into different scenarios.
For me -
An everything gun? Functioning as an EDC/CC, woods gun, varmint killer and plinker - I'd go with a 4" .357 magnum revolver. Despite being surpassed in collectability and aesthetic by guns of an older vintage, and in specific-function by semi-pistols and ARs, my S&W 686 is still my foremost companion. It caters to a wider reach of the spectrum than most other firearms I can think of - I take it with me hiking, for animals as well as the greater threat of two-legged predators; I'll take it heading out to a rural area for the weekend where I may be in public sinceI can conceal my 4" 686 under a button-down in a pinch. Plus, as the lore goes, it'll punch through a car's engine block in a pinch
I learned to reload on .38 sp and .357 mag - it's an easy platform to work with. Besides the benefits of being able to shoot two cartridges in one gun, the .38 sp can be downloaded to shoot smaller varmint, and the .357 mag can be up-loaded to shoot larger game - arguably anything in the lower 48. Not the greatest "reach out and touch from 300 yd" gun, but I'd argue the challenge of stalking, closing in and placing a precise shot is a greater character-builder.
A medium-frame .357 magnum shooting .38 specials is an ideal platform to teach new shooters on. It's heavy enough to absorb the recoil, the mechanics are simpler to grasp than a semi-auto, and first-timers always get a thrill from being able to say they shot a .357 magnum (even if they were just .38's).
However, this is a choice based on true interpretation of "only one gun."