Manyullyn makes the most useful weapons because it has the highest damage modifier (2.0).
However, as it turns out I'd been doing it wrong and cobalt has a higher base mining speed (11 vs 9) so using that for all your digging tools makes sense.
What's the point of pure ardite though?
Cobalt provides high speed, Ardite provides Stonebound 2.0. Together you get a blazing fast tool. Manyullyn is good for durability, but who really cares about being able to mine for days, I mean moss repairs your tool anyway, I'd rather be able to mine out tunnels and tunnels with ease. Also, Cobalt also gives you Reinforced 2.0 which I believe basically makes your tool as durable as if it was made out of Manyullyn. If Manyullyn actually give a modifier, it would be great, but unfortunately, it doesn't .
Edit: Yeah for weapons Manyullyn is nice, but for tools, my preference is cobalt and ardite.