Strongest practical resource you mean? What else would be a stronger offense than a barrage of nukes crippling a country for hundreds of years to come and effecting the entire global environment with radiation, fallout, complete civil destabilization, etc.

Hypothetically: your precision-guided munitions killing only family members of only military officers who haven't yet surrendered to you would be a stronger offense. Your enemy's anti-aircraft guns firing only at their own aircraft, and their tanks driving under your control to destroy their munitions depots, the crews helplessly imprisoned within. Your enemy's civilian populace becoming convinced that their own leadership is crucifying babies and mass-murdering anyone who speaks their language, your soldiers treat civilians well, and their family members are urging them to surrender to you.

Compared to those things, the power of nukes is trivial; all they can do is destroy. You can't surrender to a nuke, and the point of warfare is to convince the enemy to submit, not to damage the ecosystem. Damaging the ecosystem is just a side effect, so it's the wrong way to measure the strength of offensive resources.

Nuclear contamination doesn't tend to stay put and respect borders. Russia knows that, so I don't think Russia would drop a nuke anywhere in their own vicinity.

Correct, hence the entire reason I suggested the qualifier of practical. Nuclear attacks are clearly impractical by any sane individual, but in terms of strength and damage, aside from some sort of biological weapon, I'm not sure what could have similar impact.

Forget nuclear contamination. If Russia nukes a NATO country, nukes are coming back to land on Russia. And Putin, personally, will die, and so will Russia as a nation.

So, it was an impressive threat. I doubt he actually would follow through... but he might. That's his leverage - that we wonder if he might. Rationally, though, it's a crummy threat, because rationally his own downside is far too big.