For an all-around gun, the 4" GP.100. For one that can be carried concealed, the 3".
I've had a couple of Ruger .45s, at different times, a Convertible Blackhawk and a Vaquero. I ended up trading both away for other guns. I have a Cimmaron 4-5/8" .45 for fun. I mainly take it out with other people so they can have the fun of shooting a cowboy gun.