There is, unremarkably, a consensus here that all three are "important." There is not, however, much vagueness as to their *relative* importance. I would like to specify that what should count is importance *now*. Until we have a socialist government worthy of the name the capitalist state can be expected to stay just the same as it now is. A corrupt and corrupting plutocracy. Under plutocratic administration public regulation is to the greater part either the captive of those to be regulated or is emasculated by the horde of lobbyists suborning congress on behalf of its plutocrat masters. So the amount of atmospheric carbon reduction to be expected from regulation will be minimal. Under the plutocratic state public investment is permitted only under one form: subsidized "public/private partnership." So the public investment, though always beneficial, will be misallocated and grossly inefficient and will undergo looting by the capitalist "partners."
Carbon taxes, positive and negative (ie., subsidies for the end-users of renewable energy), avoid these problems if imposed correctly and at a sufficiently high positive starting rate, rising eventually to a prohibitive level. They cannot be avoided because they are collected directly as the coal, oil, or gas is extracted or imported. They cannot be shifted abroad because carbon-intensive imports (petrochemicals, transport fuel costs, electricity) have easily determined carbon footprints and so can be taxed at full rates. The normal workings of the capitalist market (profit maximization, cost minimization) ensure that their effects go only in the right direction, the more so the higher the (positive or negative) tax rate. The public revenues derived would provide abundant financing to mitigate completely their (as consumption taxes) regressive effect through reduction or elimination of other regressive taxes, especially the payroll tax. The demonstration effect of such a US commitment would pull, or even force, other capitalist regimes to follow.
Conclusion: Under the really-existing economic system a swingeing carbon tax is by far the most effective way to start reducing atmospheric CO2. Under a government committed to the socialist transition it would also be a way to finance the humungous public investment program required for the survival of the planet as a home for the human race and many other (though not all--coral is probably doomed) species.
But short of a drastic populist political change it ain't going to happen because it would wipe out the Saudi monarchy, the Gulf Emirates, Vladimir Putin, and most of the capital-market valuation of BP, Exxon, and their counterparts in every country.
Shane Mage "Thunderbolt steers all things." Herakleitos of Ephesos, fr. 64