Unknown but likely on both counts.
Hamas have received billions of dollars in aid for civic projects and reconstruction and used same to purchase weapons and construct defensive and offensive structures to wage war on Israel.
ISIS make millions (made millions before the drop in oil prices) per day illegally refining and selling contraband oil and oil by-products to Turkey. The illegal trade in tobacco and heroin has also been a source of large amounts of income, while nearly one billion dollars was looted from the Central Bank in Mosul when that city fell to them in 2014.
Add to that the billions from Saudi Wahhabi supporters and if they are not the richest of all time then they are in the top 5 IMHO.
Regarding their “power” – their influence and actions are global. Their campaigns international. Their recruits – cell based and lone wolf – are pervasive. And their ideology still gains traction.
The declaration of the caliphate made a universal call for Jihad beyond national and regional politics and as such created a global movement and an organisation and ideology with a reach not previously matched by any legacy terrorist organisation.
Regardless of the attitude to the declaration of the caliphate – it acts as a new departure in how radical Islam organizes and dispels with national and regional politics in favour of an over-arching unifying call to all Muslims in the context of the Mahdi and “end of days” concept.
As such its power is fundamental and has traction across all shades of Muslim opinion.
However, there are conflicting views.
Brian K. Price a twenty year and two war military veteran wrote on QUORA in response to the question that many sources do agree that ISIS is the richest terrorist organization of all time.
There are many, many other sources which compare the dollar values of every terrorist organization. ISIS’s control of oil fields in eastern Syria and norther Iraq gives it unprecedented wealth.
Is it the most powerful? That’s a tougher question to answer. As far as deadliest, most sources claim that Boko Haram is deadlier:
Though the title switches back and forth (and Boko Haram has pledged allegiance to ISIS, so there’s that…)
But does highest kill count actually mean “most powerful”? Is the point of terrorism to kill people or to achieve one’s goals?
Both groups control large swaths of territory. So that could be considered a form of power and success. Al Qaeda successfully dragged the world into Afghanistan and led to the US invasion of Iraq.
These wars have cost trillions of dollars, beyond the economic down turn that occurred in the aftermath of 9/11, the Madrid bombings, and the London Underground bombings.
Financially, I think you could argue, AQ has still done the greatest amount of damage to the world. However, AQ achieved none of its goals (Brian K. Price’s answer to Did Osama bin Laden succeed in his mission?)
Achieving political goals and gaining enduring successes should be the measure of power. And in that case, it would appear that nationalist terrorist groups have been far and away far more successful than AQ or ISIS or Boko Haram are likely to be.
Their approach has left no room for compromise which means they either win everything or they lose everything. Nationalist groups were able to force their opponents to the negotiating table and to earn concessions that continue to exist well after their violent efforts. See How Successful Is Terrorism?
Terrorist groups associated with Algeria, Cyprus, and Palestine achieved their goals of mobilized populations and varying levels of independence. Other groups such as the ETA and the IRA have attained major objectives in the form of concessions from the governing powers.
So I would argue that ISIS is not the most powerful terrorist group. It has temporarily seized a considerable amount of territory but I do not expect this will last. It will be pushed back and eventually dispersed.