Someone With Mass wrote...
Mattock before the very inaccurate Revenant any day.
And all start weapons have a better alternative.
Predator heavy pistol < Carnifex hand cannon
Shuriken SMG < Locust SMG
Avenger < Vindicator (Or the Collector rifle if you have/prefer that)
Mantis sniper rifle < Viper sniper rifle
Katana shotgun < Eviscerator
A lot of your "better alternatives" just come down to preference.
The Carnifex is much more powerful than the Predator, but it has a smaller clip and it holds fewer clips. The Predator is a fast-firing gun and can hold more clips. I find the Predator better early game than the Predator, when you have to hide behind cover more frequently and you have fewer pistol damage upgrades.
The Shuriken is a vastly underrated SMG, and is better for short-range combat than the Locust. This is good early-game for classes that do not start with a shotgun.
I wil not argue with the Vindicator being a better alternative than the Avenger. I cannot speak to the Collector Assault Rifle (I do not have it).
The Mantis is a good one-shot weapon early, perfect for Soldiers and Infiltrators until you can access the Widow. And you can still use the Mantis until the end of the game if you want (Soldiers who take the Revenant/Claymore or Infiltrator who like shotguns/assault rifles). Viper is better for every other class.
I prefer the Katana to the Eviscerator, but I know I am in the vast minority of people who like the Katana. I prefer the Katana's clip size and rate of fire more than the Eviscerator's.