It really depends on what you're doing. PvE - Fatebringer, every time. I even use it in Crota HM because the splash damage for precision kills is so good. PvP - Thorn/The Last Word, they have the highest DPS of any Handcannon in the game for PvP. Timur's Lash with Armor Pen rounds can literally 2 shot somebody though, so it's a solid choice as well. Word of Crota is lackluster in my opinion. Sure, it's great for Void Burn Nightfalls, but that's it. The Legendary perk it comes with isn't better than Fatebringer's though.

Timur's Lash, hands down. The ability to re-roll that handcannon to have the exact perks you want (My personal ones would be Zen Moment and Crowd Control w/ Field Scout) is insane. And the damage that thing puts out is ridiculous. Fatebringer is still very, very good, but it's just being outclassed damage wise. (Arc Burn not included)

I can't really answer that, never really use handcannons