I'd say you gotta have these food groups represented. I'll list the Blue model number, but it could just as well be the stainless counterpart:
K frame rimfire. Model 17 or 18
K frame in 357 or 38. Model 19 or 10
J frame. Model 36
N frame in 357 magnum. Model 27/28
N frame in 44 magnum. Model 29.
I'd say once you have a variant of these, you can enjoy a wide range of shooting experiences. From there, you'd probably want a 586. And from there, you can start broadening into different calibers, barrel lengths and finishes.
But it's a highly personal thing. Some guys would say you gotta have different eras represented, for example. You can't argue with that.
For me, I think a 4" K frame is indispensable from any collection. If I could only have one, I think it would be my model 66. It's the one that does the most things well.