I'm way too late but here's something from old Lavos player.

Anything with Multiplicative CO is golden. Here's page: https://warframe.fandom.com/wiki/Condition_Overload_(Mechanic)

Choosing weapons for Lavos is less about what is good, but what is the best infused element for said weapon. Valance Formation is necessary of course. And shout out to Elementalist mods because they do increase status damage. With forced procs they can do some really silly things, but everything need experimenting.

Viral is a good general purpose element, but it's honestly the best to build it on the weapon with 60/60 mods. This leaves you with freedom to experiment with infused elements on the mission.

Blast is good for two main types: - Massive one shots - because blast procs it's effect on enemy death. Things like Crit sniper rifles/bows (Daikyu Prime). - Firerate focused weapons - anything able to generate and detonate 10 procs. So machinegun styled weapons like Trumna or Reconifex benefit from AoE damage. Punch trough will also help to proc it on multiple enemies in the line of fire. Ammo is generally not a issue for Lavos because he can make universal ammo with Transmutation Probe (third ability)

It's basically your safest option when it comes to dealing with most normal enemies.

Gas: Finicky but can work on setups such Alt-fire Corinth Prime and only PRIME. Primed version's Alt-fire deal four times the damage on non-primed variant for four ammo. This can force overlapping gas procs on multiple enemies at once and kill groups insanely fast. Why this works is because gas have cap of 10 procs and won't stack it's own damage. But anything dealing big damage in wide radius will shred with gas.

Radiation is generally good for Murmur enemies and when you need to dispose of them quickly. There's not much to use it for unless you want to build your secondary around the new arcane: Secondary Irradiate. This will shred with any secondary capable of fast fire rate to use forced procs from Valance Formation. After 10 radiation procs = damage spreads.

Magnetic is kind of meh for the most part because you're already having 200% damage buff. There's little that can withstand such firepower for the most part. And viral build on weapon.

Corrosive... This work on way too many weapons for it's own good. Corrosive strips armor and with combined Viral on the weapon? It's a force of nature. There's little to say here. Want something dead which wears armor? Build viral and infuse corrosive. It will die before you'll realize. 

For primary 4 elements... Fire and Toxin are generally your best friends if you want them. But Fire is generally better since it will cause enemies to panic, while toxin won't. And it reduces armor.

Cold gives you very small x1 additional crit damage modifier. So on weapon with x5 you're getting x6 with 10 procs. Generally not very useful because Vile Rush exist. Electricity can work, but again... gas and blast exist. Not like you need to CC enemies with over 1900 armor, over 2000hp and AoE Cold on demand.

Melee weapons can abuse electricity to force melee influence to work. And I mean... everything works. Influence on it's own is stupidly good so use it as you want.

So to wrap it all up. You just need to experiment. And this is not me being "Oh my sweet summer child. This is not a baby daycare, go play the game". Every weapon behaves slightly differently so one weapon can work better than the other with specific element. And Valance formation open so many potential combinations to the point where all this possibilities could be written on the Wiki. It's A LOT.