Tom Bosley
I have not watched this delightful film since its initial release, when I was too young to appreciate the deeper levels of what was going on and was only mildly captured by the humorous aspects. Still, something about this film has stayed with me years later and I have just enjoyed experiencing it all over again on TCM.
Along with all the praises that have been enumerated, including terrific performances from Tippy Walker, Merrie Spaeth, and the principles, smart writing and sensitive direction, how about a hand for an understated and moving performance from Tom Bosely, which makes the entire finale work. It's rather a shame that this fine actor (see also "Night Gallery" pilot) will likely be remembered only as Richie Cunningham's dad on "Happy Days."