Depends of it's use so a variety is needed
A revolver collection should cover target practice, plinking, varmint shooting, home protection, concealed carry and bear defense. I nominate the following with that criteria in mind:
1. Smith & Wesson 686+ 4" bbl. S&W revolvers are the best and I like having at least one revolver that can hold an extra round.
2. Smith and Wesson Model 29 44mag. If good enough for Harry Callahan, it's good enough for me.
3. Ruger Single Six (You choose the barrel length)
4. Colt Python, haven't shot one but with it's reputation...
5. Smith & Wesson J-frame snubbie in 38spl.
The 5 revolvers cover most above covers most needs but I wish the list could have had 7 because It's a pity I couldn't include the Colt 45cal 1873 Peacemaker with a 5.5 bbl and a 6" Smith & Wesson double action revolver in 22LR such a model 617 or 17.