For most builds, yes, there are better Legendary effects that the .44 Pistol can have. However, for a Gunslinger Crit build, it's easily the best weapon you can get. First off, it's a .44 pistol, which has the highest damage of any pistol-grip weapon. So in terms of damage per shot, you're not going to find better. Second, it comes with the Relentless Legendary effect, which refills your Action Points on a Crit. Because Crit builds are obviously doing crits quite often, this makes the Relentless effect an excellent and reliable way to keep their Action Points full; other builds have to rely on Grim Reaper's Sprint, which only has a 35% chance to proc on a kill. Since Kellogg's Pistol is also a guaranteed drop from a specific enemy, it's also easy to get. This makes it a great weapon for a Gunslinger Crit build.