What are some must have mods for primary and secondary weapons?

[Rivyn]

Title. I've always preferred melee, but I'm looking to change that up. 

[NinjaZeku]

Base Damage, Multishot and Elemental Damage Mods have a place on pretty much any weapon.

Then there's Crit Chance & Multiplier stuff if the weapon supports building for that,
Punch Through can be neat, Fire Rate often makes a huge difference in DPS (and usability of Charge weapons), ...

Maybe simply take a look at some of the builds on Warframe Builder to see which Mods keep popping up,
then check the Wiki on how to acquire the Mods in question if you're lacking some of 'em.

[(PSN)HammerOfTheMist]

As mentioned, warframe-builder is good.  Also this: https://www.reddit.com/r/Warframe/comments/2t4fms/what_are_standard_or_essential_mods/

You probably already know this, but generally you'll want to mod for what the weapon is good at:

• Damage increase and multishot are always there (e.g., Serration and Split-Chamber for rifles)
• Crit weapons (my personal rule of thumb is if crit chance >= 25%, it's a crit weapon - e.g., Dread).  Max Point Strike & Vital Sense and use the 90% elemental damage mods and Hammer Shot.
• Status weapons (again, my personal rule is both status chance >=20% and high fire rate).  Use the dual elemental-damage/status mods and Hammer Shot
• Hybrid weapons (both crit and status chance are close the above - e.g., Grakata or Zhuge).  Use Point Strike and Vital Sense and the dual elemental-damage/status mods and Hammer Shot
• Failing all of the above checks, the weapon is pure IPS damage (e.g., Karak), so use the 90% elemental mods, Heavy Caliber, and one of the IPS damage boosters if the weapon's damage come primarily from just one of the three IPS types. 
• Leftover slots would go to 'quality of life' mods (fire rate, recoil reduction, bane of XXX, etc.).

These are of course my own personal guidelines and I'm sure many will offer alternate opinions.

I will add that while the warframe-builder website is very good, I don't think it accounts for status procs.  For example, a viral proc will temporarily reduce the target's health to half and both impact and elec procs will stun enemies (preventing them from shooting back).  I've got a Grakata I've got setup for both crit and viral + elec procs.  And even though it's stats are underwhelming It does fine on most everything on the star chart.

And above all, find some weapons that are fun - I'll put in a plug for the Sonicor as I just love that thing.

Good luck, Tenno.

[MrThumpy]

All of the above is very detailed advice, but just to make it complete, since you asked for concrete mods, which are must-have... (going from memory, so sorry for any inaccuracies)

 

Primary: Serration (damage), Split Chamber (multishot), Heavy Caliber (+damage -accuracy ... many weapons benefit from this)

 - in case of crit weapons (see the above post) Point Strike (crit chance), Vital Sense (crit damage)

 

Secondary: Hornet Strike (damage), Barrel Difusion (multishot), Lethal Torrent (multishot and fire rate), optinally Magnum Force (+damage -accuracy ... not many people use this on their secondaries, but it works)

 - again in case of crit viable weapons: Pistol Gambit (crit chance), Target Cracker (crit damage) ... both of these have a primed variant, so if you like any crit viable secondaries, I would suggest you visit Baro (the void trader) asap.

 

90% Elemental mods and 60% dual stat mods for both primary and secondary weapons are all must have so I wont list them here one by one, but they are mostly easy to aquire so no worries.

Also dont be affraid to experiment. Most awesome weapon builds were discovered by stepping oustide the "meta zone".

See you on the battle field, Tenno!

[(PSN)HSomDevil]

9 hours ago, (PS4)HammerOfTheMist said:

• Crit weapons (my personal rule of thumb is if crit chance >= 25%, it's a crit weapon - e.g., Dread).  Max Point Strike & Vital Sense and use the 90% elemental damage mods and Hammer Shot.
• Status weapons (again, my personal rule is both status chance >=20% and high fire rate).  Use the dual elemental-damage/status mods and Hammer Shot

Just to follow up:

I could start a debate whether it's worth having Hammer shot on anything, ever.

But since OP was asking about "must have" mods, Hammer shot most certainly isn't one.

Otherwise I endorse that list. 

[NinjaZeku]

40 minutes ago, MrThumpy said:

optinally Magnum Force (+damage -accuracy ... not many people use this on their secondaries, but it works)

It works, sure, but it's generally a pretty bad choice over e.g. another Elemental Damage or "just" a QoL Mod.

Heavy Caliber, this is not.