I agree with all the foregoing, depends on what for. I have a 4" redhawk (45 Colt) as a hunting sidearm, a 3.75" New Vaquero for CCW.
I used to have a 7.5" revolver that I bought for hunting, but I couldn't shoot very far with it, and I couldn't get it into action quick enough to think of it as a bear defense pistol either, so I went back to 4" sidearm and back to toting a rifle.
For sheer looks, I like the 4.75" Colt SAA. It's next on my list.