@Tom Tee posted:HO or N
Bite your tongue! 
You're right of course. In all seriousness I've struggled with this issue first-hand. I've come to believe that in many ways the size of your room (along with your operating preferences) dictates what your max curve should be.
For years I collected "scale" (hi-rail) trains, with hopes of building a "big layout." When I went to buy a house, reality hit me hard. (I live in the Land of No Basements.) My train room is 11 x 18, and living in the Sun Belt I doubt I'll ever have more than that. Yes, I could do concentric ovals of O72 and O81. This would allow me to run anything, but it leaves only 10' for straightaways. That's not much when you consider that a Big Boy, A-B-A set, etc. are over 3 feet long, it doesn't leave much for the train! Some people would be thrilled with a setup like this, because all they really want is a stage to display their locos in motion, with a bit of scenery along the right-of-way to provide some context. But I stopped building that layout because I was afraid I would be bored with it before the paint was dry.
I want my railroad to be a highly-functional integrated system, not only a stage to display my locos in motion. So it's tough for me to justify "a loop of O72 around the outside" just to occasionally run the Big Boy in circles. I kept thinking of how that space would be better used to create alternate routes, sidings, yard, etc. The only solution, a radical one, was to move away from scale and back into "traditional" O gauge. Maybe Lionel was onto something when they re-emerged into the postwar world of 1946? I hope the sharper curves will give me the operating variety that I crave. Thankfully, with smaller Legacy locos, RailKing, and LionChief Plus, sharp curves and smooth operation at slow speeds are no longer mutually exclusive! And the smaller locos will look just as good on O42 as the larger ones would have on O72.
During this time of soul-searching I also realized that I'm not a scale modeler. I've never weathered, or ballasted, or painted anything in my life! So with more room the trains might be scale-sized, but the overall result wouldn't be any more realistic than Postwar running through Plasticville. Like many of you, I grew up with that, and it inspires my imagination to this day.
There's no one best way to enjoy this hobby. But geometry is a cruel master. Wider curves mean shorter straights, and I do believe that the size of your room strongly suggests your max curve, train length, etc. My $.02, worthy subject!
