Hard to say. On one hand, Void seems far more fundamental of a force than “Fel”, that discrepancy evens out somewhat if you think of “Fel” as “Chaos”. Which was a far more explicit connection in earlier WarCraft games (Reign of Chaos, not Fel, Chaos Orcs not Fel, etc.)
Still, Void is still more fundamental, as it was the second “thing” to exist other than the Light.
That said… Addiction? While they’re both addictive to an extent, it’s more associated with the Fel. Power is addicting, and the Fel leans stronger on that notion.
For destruction, it would be Fel simply because Fel is Destruction. Destruction for destructions’s sake, destruction for the fun of it - it feeds off the idea. Void, however, feeds by… well, feeding. Feeding requires preparation, effort - you can set a field of crops on fire and destroy it faster than you can harvest, cook, and eat it. The Old Gods and all their machinations are really just a ridiculously long dinner preparation for the Void, while the Demons of the Legion in the destruction they wreck are skipping right to the end goal. Yes, Sargeras does have a “plan” and a “goal” but your rank and file Felguard and Pitfiend only sees that as an excuse to do what they would happily be doing anyways.
As for power… That’s really hard to say because the Void is so incredibly constrained. On even ground I’d put the Void and the Void Lords over the whole of the Nether and Sargeras himself, but… there is no such even ground. The whole idea behind the Old Gods, arguably the linchpin of the whole plot is that the Void and Void Lords can’t stand on even ground with … anything.
They absolutely, 100% can’t leave the Void. That’s not negotiable, so any hypothetical contest where that’s taken out is entirely invalid. Their hands/tentacles are tied in ways they can only weasel around by spitting the Old Gods into the depths of space and hoping for the one in a squillion chance their googly eyed offspring hit a planet with a fetal titan in it.
The Fel, meanwhile, is constrained only by access, which is the same for just about any other power. Underneath Sargeras, the Burning Legion destroyed “countless worlds” while the Old Gods… we have no reliable source on those numbers. But a raw form of power, which has higher potential? …
I’d hesitantly give that crown to the Fel because the most powerful Fel converts are always far greater than those of the Void and even the vast majority of natural Fel Creatures, while the Void’s greatest powers are always spawned of the Void. You could theoretically become the next Archimonde or Kil’jaeden, but there’s nothing that could make you the next N’zoth.