its hard to say, they are as worst in my opinion for this reason:
Subban, Weber:
Montreal knew exacly what kind of talent they had in Subban and Weber was an obvious downgrade at the very moment and even more in the future ( i dont care what anyone thinks, its the truth, defensively + offensively Subban is superior.)
Forsberg, erat trade:
Obviously forsberg was just drafted and he had potential but it was still unknown at that moment and no one would have predict he would become this good. Washington were desperate for immediate help to support the group of foward for a playoff push, erat was obsiously not the best guy, but was probably their best option at that very moment and from what i heard the owner or someone in the organisation didnt like forsberg.
so yeah to be fair if you check Player for player right now the forsberg trade hurts more and its the worst of both. but if you consider just the trade, forsberg was nothing but a prospect and not proven, The Subban trade is worst because you knew what you were trading and what kind of contract and player you were getting. yes for some people its arguable, because some people see Weber as a god, but he isnt has relevant and useful like Subban in todays NHL where mobility and quick action matters more than pushing people in front of the net and having a big blast.( to be fair, Weber is not even ****ing physical! at least he isnt anymore.) so to me the the Subban trade is worst because you were suppose to have that knowledge.