The USA. It has the capability to strike anywhere and it has also shown its willingness to strike. China could be more dangerous but military-wise, it seems to stick to itself.

Well depends what we look at really:

NUKES = USA / RUSSIA and CHINA MAYBE?
INTERNAL ISSUES = SYRIA, SOUTH SUDAN, BRAZIL
DANGEROUS ANIMALS = AUSTRALIA
UNSTABLE LEADERS = USA / RUSSIA / NORTH KOREA

Scotland!

Right now, the US, China, Russia, North Korea, India, Pakistan, and Iran are the major contenders for dangerous governments, particularly with that caveat of "if they wanted to" because the US is fairly stable in spite of the current leader but if the worst of the current administration truly gained control with no checks and balances whatsoever, seizing all power in a true dictatorship with malevolent intentions, the US simply has the most firepower. We could take on most of the world together all by ourselves. I'd genuinely bet that if the US wanted to, it could handle the combined might of all of Europe, Africa, South America, and the Middle East if Australia, Mexico, Canada, and all Asian countries stayed neutral. That is a disturbing amount of power for one country to hold, no matter how stable it is.

In terms of a more objective threat to life on Earth, Russia is probably the most dangerous, because it's just so unstable right now. It appears stable with Putin, but one 66 year old man in charge of a kleptocracy with multiple factions at odds with each other and the world is not what I'd want to rest the stability of the country with the most numerous and powerful nuclear arsenal in the world right before it withdraws from an arms reduction treaty and begins a new arms race. If Putin dies suddenly, the world is fucked. Russia will not stay united, it will split along various lines of power and not all the tens of thousands of nukes would be in the hands of sane men. There are some terrifying anti-Western radicals whose paranoia outclasses some of the West's worst culprits in the conspiracist right. They absolutely would launch nukes preemptively if given the chance, and would be easily provoked into doing so. They make Kim Jong Un and Trump's saber rattling recently look tame and unalarming by comparison. And at least some of Russia's nukes will end up in their hands if Putin dies suddenly without clearly grooming a successor.

China is certainly a threat to the current world order of western civilization holding most of world power, but I don't think they're a danger to life on Earth. They're just fairly pragmatic and nationalistic. That makes them very dangerous to be sure, but not as catastrophically so as Russia would be. If Xi Jinping suddenly died without a clear successor, China too would be at risk of breaking apart, but it would be along much safer lines, with Hong Kong and Taiwan and maybe even Tibet standing a chance at true independence, which would be nice. Meanwhile having several Communist China factions as rivals to control what was left would be good for the rest of the world. Some are very anti-Western, but none of them are blow-up-the-world-on-a-whim crazy like some of Russia's oligarchs.

North Korea is a bit of a wild card because of their relationship with the US and China's firm support of them, as well as Russia's occasional dealings with them. That creates nuclear instability, but if China wanted the Kim regime gone, it could be done safely for all countries. Iran is pretty nuts right now and honestly a bit scarier to me than North Korea, because their theocratic government is completely nuts in a way only a religious government could be. It's basically a slightly more stable, slightly more pragmatic, much more legitimized, and not quite as viscerally horrifying Islamic State. What makes it truly scary is that it really could become a nuclear power, and I'd sooner trust Kim Jong Un with nukes than Iran's Supreme Leader.

India and Pakistan are dangerous to the whole world because of their relationship to each other. If I were a betting man, and I knew Putin was going to live long enough to install his successor, and I had to bet who would start the first nuclear war, I'd put good money on the conflict between India and Pakistan being where WWIII starts, at least at this point. Conflict is escalating, there's an entanglement of alliances much like in the previous world wars that could drag more combatants into the conflict, and both are nuclear powers. Also Pakistan is full of terrorists and there's no telling if their nukes are really all that secure. Ours sure aren't in the US, so I doubt Pakistan's are either. One of those terrorists could get ahold of a nuke and launch it at India or even the US (though we might be able to stop it if it were just a few of them). The best we could hope for in a nuclear conflict between the two is that all alliances are severed with both of them to avoid escalation, and that the conflict is contained just just 100 nukes or so, causing a small nuclear winter that the planet could just barely survive if we worked together and only India and Pakistan had sustained much damage to major infrastructure.

Currently the US.

Russia projects its power near itself, but overall they aren't big moves. A small country or province here and there.

China doesn't project its power unless it is in "China". So China Sea and within China or its borders like with India.

USA often projects its power halfway across the world. If a country becomes unstable and the people in power of the US thinks it is a good idea to invade, then they will. Sometimes decently sizeable countries like Afghanistan and Iraq.

Australia and Canada

And this is why I left it behind. Been kidnaped twice in sao paulo. My parents had their house invaded with them inside.  Crack addicts everywhere. Had enough of it.

Lol what about Luxembourg, San Marino, Andora, they are even smaller.

Feel really sorry to hear about it.

With regards to countries that are a danger to other countries I would say it is any country with nukes and similar weapons of mass destruction, a large armed forces, and/or a large military industrial complex/a government that seeks legitimacy through military might or hostile expansionism.

To me this comes down to a list of countries:
A-tier dangerous
-USA
-China
-Russia
-North Korea
-Democratic Republic of Congo (going through a domestic armed conflict during an Ebola outbreak that is not being controlled and could spread to other countries; if it was not for the uncontrolled Ebola outbreak this country would not be on this list)

B-tier dangerous
-Saudi Arabia
-Iran
-Israel

C-tier dangerous
-India
-Pakistan

As for countries that are the most dangerous to live in they are typically ones that are war-torn, impacted by political/gang/cartel violence, have high violent crime rates, and/or are fighting deadly infectious disease etc.

My list of such countries would include:
-Democratic Republic of Congo (going through a domestic armed conflict while an uncontrolled Ebola outbreak is happening)
-Yemen (has totally been demonstrated by the US/Saudi and Iran/Russia proxy war)
-Syria (same as Yemen)
-Afghanistan (was being modernized by Zahir Shah and Daoud Khan; but the Shah's overthrow by Khan and Khan's subsequent assassination was the start of the country's devastation)
-Saudi Arabia (threat of government violence and harsh control of society by the government; women, minorities, political critics are especially vulnerable)
-North Korea(threat of government violence and harsh control of society by the government)
-Central African Republic (civil war, ethnic violence)
-Libya (as a result of the second Libyan Civil War)
-Somalia (insurgency, political conflict)
-South Sudan (civil war)
-Sudan (civil war in the south, and harsh government control)
-Mexico (high crime rate, cartels)
-Brazil (political unrest, high crime rate)
-Venezuela (unrest due to political unrest and severe economic downturn)
-Colombia (same as Mexico)
*I should mention this list is based on the general situation of the country. More specific populations like various religious/ethnic groups, political groups, LGBTQ individuals, etc. might face disproportionate violence in countries that I have not listed as being generally dangerous.