I assume you're interested in shooting in Stock Service Revolver, where the gun is speedloader-fed? A moonclipped gun with put you into Enhanced Service Revolver, which requires a Power Factor that essentially means a .45acp Model 625.
The L-frame 686 with a full underlug is probably the single most popular choice of gun. K-frame .38spls (e.g. models 10, 15, 64, 67) or .357mags (e.g. models 13, 19, 65, 66). Of the k-frames, I'd opt for a model with adjustable sights (15, 67, 19 or 66), though that's not absolutely necessary.
The 686SSR seems very popular, and if you're going to leave the gun stock, is an option worth considering. At the very least, IIRC, it comes with interchangeable sights, so you can easily swap in a fiber optic front sight, which is very nice to have.
The Ruger GP100 also appears here and there. It tunes up very nicely.
Remember, there's a 4" barrel length limit.
Once you get your gun, get some Safariland CompIII speedloaders or JetLoaders, some dummy rounds, and start practicing your reloads!