Hearthpwn

The set with the highest possible power level has to be Mean Streets of Gadgetzan. It pushed reno decks to a new level, introduced Jades and propelled Reno decks to the epitome of their strength in standard.

But if that isn't convincing, then we can go off the single card that sees play in every single aggro deck and will continue to see play in wild until Hearthstone gets surpassed ~ Patches the Pirate!

Msog for Patches 

When you ask what was the strongest set, we have to consider the filler cards. MSG maybe had some stronger cards than Naxxramas (assuming we consider Undertaker to be pre-nerf), but it had both more cards, which I think should be considered, and it had more filler cards. Naxxramas is in top three of the expansions that would have the biggest impact, and it's definitely the number one in the average power level, and it did that with a fifth or so of the card amount of full expansions. There isn't any other expansion to consider.

Quote from DoubleSummon >>

Quote from thetrailofthedead >>

I guess a lot of people weren't around in the days when naxx came out but that set is straight up broken.

Zombie chow, haunted creeper, mad scientist, Loatheb, sludge belcher, void caller, deaths bite and of course,  under taker

Half of the set would still be auto include in their respective deck types

 you can see in wild this isn't the case.

If we are considering undertaker prenerf than we must consider smalltime buccaneer pre nerf too... 

Naxxramas is a close second, but msog was straight up broken. 

I think Knights was the strongest set until now, but MSOG has around the same powerlevel. Since Knights enabled many MSOG-Decks, i count many MSOG-Cards that are viable right now (like Priest) to Knights. But Patches alone (and especially the total op 1 mana 3/2 that was nerfed) made MSOG an insane powercreep-expansion. But Knights managed one thing that didn't happened in the entire history of hearthstone: establish a control-heavy meta.

So for me its:

1.) Knights
2.) MSOG
3.) Naxxramas
4.) GvG

Control heavy what? All i saw was keleseth zoo and machine gun priest. 

Both playable thanks to cards out of msog. 

TGT. We got bolf

Naxxramas by far if we're speaking relative to launch time. Imo, league of explorers enabled the strongest aggro decks we've seen short of pre-nerf patches pirate warrior. 

By far the strongest set to be ever released was MSOG,it was the most meta-defining and the only set to ever come close to it has to be K&C .