Perennially favorite revolver topic for discussion, but one that usually doesn't produce any answers that are universally agreeable.
Being a long time revolver owner/shooter, I'd have to first consider the roles for which I was going to need filled by a revolver.
This would include not only the specific activities, but the tasks anticipated needing to be done during those activities, and the locations in which those activities (and tasks) might occur.
The manner of carry (openly or concealed) would also have an influence.
Depending on the circumstances and situations I've experienced, I could easily make an argument for a .44 Magnum, a .357 Magnum or a 5-shot J-frame snub being the "best" compromise for my needs at various times.
This sort of thing could be further broken down by the basic design of the revolver, meaning traditional double action (capable of trigger cocking or thumb cocking), or a single action design (only thumb cocking).
Tell me where I'm going to live for the rest of my life, under what conditions, the most commonly expected role for which the revolver would be needed and the expected carry method (required or allowed) ... and I could choose the best compromise in design, caliber and size.
That's why I still own both DA & SA revolvers, covering .38 Spl, .357 Mag, .44 Mag & .45 Colt, and randing in size from 5-shot snubs to mid-size & full-size (N-frame, Redhawk & SBH).
Now, for a light-duty, medium-bore caliber, I agree it would be hard to beat a M65 3-4".
For something more durable, over the long haul, I'd leave the K-frame in the safe and choose my Ruger Service-Six 4" HB .357, though.