Unfortunately there is no "best" or that's all everyone would buy. We all have varying tastes and needs.
If target shooting is the primary use for the gun, then a 6" would be an excellent choice. But for most other applications, I find a 6" to be much too long, cumbersome,and unbalanced myself. Fortunately there are a lot of different frame and barrel configurations available, 'cause no one gun could possibly even come close to doing it all.
My 6" 686 is my least used (and least maneuverable) of 17 or 18 S&W and Ruger DA revolvers that I have. Of course I don't spend much of my shooting time doing long distance precision target shooting. Most of my shooting practice is close to mid range (ten to 25 feet), double action, quick moving defensive type shooting... not sloooow, careful squeezing for precision placement at great distances. Yes, it's a well made and accurate gun and I do enjoy shooting it once in a while, but for me, I much prefer the feel, weight, maneuverability, and balance of a 3" K-frame or GP-100 the most, followed by a 4" GP-100 or K-/L-frame. Quick moves from target to target is much more difficult with a 6" barrel than with a 3 or 4", though the added weight helps a little in keeping the nose down.
Fortunately (or unfortunately, depending on how you look at it) no single gun is the best for everything, not necessarily even the best for any one application, much less several. I have distinct use and preferences for a 2" 624, 3" SP-101, 3" GP-100, 3" K-frame models, 4" GP and L-models, and, finally, 6" models. All depends on the intended use.
Go to a gun shop and hold these guns to get a feel for them. That will help you decide.
If you want one gun to try to cover numerous needs, my recommendation would be a 4" GP-100 with adjustable sights. Not ideal at any one thing, but a 4" revolver is great for general use and passable for some other uses. Home defense, target shooting, camping/hiking, plinking, it'll do fine. Long distance/precision accuracy work, it's ok, but the 6" is easier to be accurate with due to the longer sighting radius.
Carry/defense duty a 3" is ideal; 4" so-so to ok; 6" wacky. Deep concealment/carry gun go with a small frame 2" or 3" J-frame or SP-101.
What I'm getting at is that you need to decide what you reaaaally want to do with this gun and pick one which does that especially well... then plan on buying another one or two (or three or more) to cover your other bases properly. But if you've got to stick pretty much with one gun... make it a 3" if carry & defense is more important or a 4" if defense & target is more important.
My favorite GP-100 (I have three of them) is this one:
Ruger GP-100 3" Barrel, Blue Steel, Fixed sights, .357 magnum (GPF-331)
Feel in the hand, weight, size, comfort, shootability, durability, poiintability, concealability, pack-ability, and defense accuracy for close to mid-range use is superb. Puts a big smile on my face every time I handle it.
