The optimal build for most Crit-viable Kuva/Tenet Rifles.
What distinguishes Kuva/Tenet weapons from others is that they all have an innate Elemental bonus, potentially opening up a mod slot for builds with combined Elementals.
For Rifles the bonus of choice is Toxin, to make the Primed Cold mod combine into Viral.
Heat Bonus and 60/60Viral+Heat mods are better for DPS vs unarmored enemies, but with Armor being as prevalent as it is, building for unarmored enemies is very niche.
Primed Cold (Cryo Rounds) is always better than 60/60Cold for weapons without significant innate Slash, even when you count the damage increase from the +SC through Viral, so you'll want Primed Cold on most of your Rifle builds.
Replace Faction(Bane) mod with Galvanized Scope for a more good-vs-all-factions build. I'm generally not a fan of highly conditional mods like that tho.
Riven
Riven stat priority: CC = CD > MS = FR > Dmg
CC > CD if below ~90% final CC cuz increased HunterMunitions reliability, else CD > CC.
FR > MS if you wanna use a Faction(Bane) mod(Riven replaces VileAccel), else MS > FR.
Riven replaces mod with same stats or Faction(Bane) for a generic build. But don't replace Galvanized Chamber even if Riven has MS.
Arcanes
Merciless is the easy all-round option that is especially effective on low-ammo and weapons with low shooting:reload ratios.
Deadhead is the more niche option for precision weapons where recoil might be an issue, or where the headshot multiplier bonus can be abused.
Dexterity is mainly useful as a stat-stick for your melee by increasing Combo Duration without taking up a mod slot in your Melee weapon.
Good Exilus mods
Vigilante Supplies - The only option that provides a tiny increase to your damage, while also bringing you extra ammo.
Stabilizer - Nice for weapons where Recoil is an issue.
Twitch / Tactical Reload - Nice utility.
Terminal Velocity - Good for weapons with damage-falloff, but useless on all others.