Even though they have rather different stats, all the Revolvers can be useful in my opinion.
Regular Revolver is overall average, since its fire-power is alright, but not to be used as a focusing weapon. I say use it as a last-resort weapon when finishing off foes from a distance.
Ambassador is a trade-off for accuracy, so it's nice to use if your a good shot. The kind of situation you could use it in is a possible cornered situation (something similar to a Revolver situation).
L'Etranger kind of feels like a gimmick weapon in my opinion, due to the cloak boost being the only real perk of it. I suppose that's not really a bad thing, since it can be used well (just not often from what I've seen). You can use it to get back your DR and IV charge back, so at least it has that.
Enforcer would most likely be better used a finish-off weapon because of its DPS increase for killing off enemies. The slower firing speed doesn't really effect the perk of this weapon from what I've seen.
Diamondback should be useful for Gunslinger Engineers, with their low-health Mini Sentries. As for Regular Buildings and Sentries, I suppose it can come in handy with the Stock Sapper. Relating to stats, it's basically the Ambassador minus the Headshot Perk.
Final recommendation: I say Ambassador or Enforcer would be the most useful, but I prefer all of them.