Bit of propaganda for ya. Previously, Toxin was considered the king, because we had to deal with Corpus with bugged, excessive shields and Grineer with thousands of times of damage mitigation, so raw tox and viral/slash were the kings.
Magnetic was always a bit of a sleeper lazy pick, as it was a free multiplier for Galv Aptitude/Savvy/Shot and covered Corpus for free while letting you build for Grineer.
Nowadays, while the above is still true, Magnetic enjoys significant improvements. It now applies vulnerability to Overguard, meaning it covers Viral’s worst weakness. In addition, breaking Overguard while an enemy has a mag proc on them turns them into a bomb, by applying an Electric proc based on their max HP and scaled off status damage mods and damage vulnerability effects. Finally, melee enjoys the flexibility to do full-power builds around Melee Vortex which is fairly considerable as an alternative to Melee Influence; in addition Melee Exposure gives you good universal coverage totally for free.
So what would I recommend as the #2? For melees, Electric is a shoe in for Influence usage as it improves the probability (but not power) of getting electric procs. For everything else, buckle up, I’m doing a screed on Cold now.
Cold has some very desirable properties and one very considerable flaw. What those properties are changes based on slot, but both Primary and Secondary have arcanes to facilitate Cold.
Before that though, Cold offers a lot of flexibility. On high proc rate weapons, it offers Frostbite for free, as well as Cold’s innate crit buffs. Otherwise, it’s a pivot element for Blast, which is a very powerful universal element at the moment, Viral and Magnetic.
As for arcane support, we have Primary Frostbite, which is usually competitive with acolyte arcanes and actually exceeds them under certain buffing conditions. Secondaries benefit from three main options: Secondary Shiver, which functions like Galvanized Shot, but with cold stack count instead of unique status count, Cascadia Accuracy, which exploits the pringle maths of crit scaling to get outsized rewards, and Secondary Outburst for the same reason. It’s worth noting that Secondary Shiver also enjoys multiplicative scaling on certain projectile options, for instance the Cyanex.
Now, what’s that flaw I mentioned with Cold? Its complementary element selection is poor for damage. Corrosive will be the typical companion element, but you also have access to Gas, or modding directly for Radiation if you choose not to use a radiation mod. Gas is ultimately a poor substitute for Blast unless you are able to support it with damage vulnerability or fullstrip (or both like a certain sus sentient frame).
I personally will continue to go for Magnetic just because I like the versatility of a non-combining secondary element, but with the announced Coda shop system I’m always going to be happy to grab a Cold alternative while I wait.
Anyway, this is not to lecture you, dear reader, not to take Tox progenitors. I just want to lay out that we have very strong options now.