If you’re using the Kuva Nukor for priming, then Magnetic is probably the best for that one. Adding Magnetic or Impact lets you get one more status from the weapon than other choices. Magnetic’s status effect is likely more desirable and significantly more weapons have impact than magnetic – so the weapon being primed for could occasionally add another status if your primer isn’t also adding impact.

I generally use Heat if I don’t have a specific use. You can then still create viral+heat or corrosive+heat with only 2 mods, or gas with only 1.

Toxin is very popular since you can create viral with only 1 mod. But if you want viral+heat, then you still need to use 3 mods because the added element from liches is applied after elements from mods… so if you want that combination then heat would be better.

I’ve also been enjoying Electricity when using full armour strip lately.

Honestly, the only one I never use is Radiation.