If you use any build other than these you are wrong and should feel bad.
This website has my favorite builds for each class. Each build shows two possible weapon combos, with the top combo being my preferred combo that I actually use in game. The screenshot of the reject on the right is a screenshot of my cosmetics for the class.
Everyone knows that each loadout should look different from all the others. I also use different weapons on each loadout that don't duplicate with any other builds so I have the most variety possible. I also try to select each grenade and active ability to further change class.
I don't duplicate any weapons for any loadout, and I make sure every Blitz, Ability, and Keystone are used by at least one build. Due to this many loadouts are not 100% optimal. As such sometimes the bottom weapon combo is the "better" weapon combo, even though I don't use it. All loadouts are viable on any difficuly of the game, but some are easier than others.
The loadouts are listed in alphabetical order, rather than based on how effective/fun they are. I I have a * by the loadout name for the easiest loadouts. These loadouts are mostly optimal already, without any tweaks. Some of the other builds could be more optimal, but have "unoptimal" nodes to introduce more variety.
Some of the builds descriptions have a new line starting with an *, which include thoughts for how to change the build for other playstyles.
There is an excellent guide from Reddit user /u/CommissarTyr that goes over every damage type each enemy has. Many enemies in this game have multiple damage types, depending on where you hit them. There are also different damage multipliers, depending on where you hit each enemy. Visit it HERE.
Another obvious resource is the Meat Grinder inside the game. It really lets you test your weapon breakpoints to test what type of modifiers you should bring on them.
This website was last updated for the Unlocked and Loaded update. If you want to see my builds for older versions of the game for some reason CLICK HERE.