Which role is the most toxic now? Let me give you my vision on this and then give me yours.
As I see it, toxicity distributes like this:
**Supp < Mid < Top < Jungle < ADC**
How do I calculate this? By how many people can blame you, and how much you can do about this.
**Top** lane will be my reference. It is the most average and flame-balanced position. On one hand Top is a kind of "quiet harbor". You may 1 vs 1 for a long time, keeping balance and afk-farming. On the other hand you may get a lot of flame lategame in case you lose this 1 vs 1 and feed your opponent. Also sometimes your top may turn into a ganking contest which will make things somewhat hard for you. Still, you will mostly get a quiet early game and a chance to get some real help in case you fall behind. Blame to take and influence are balanced.
Now if we start to compare... The nearest options would be Mid and jungle.
**Mid** is easier than top in terms of blaming. You've got both flanks open for enemy jungle to gank, thats' true, but the same is true for your opponent.
And in case you run into a bad opponent, you may always go roam and say it's not your fault. Just try to stay above 0/2 (0/3 at the most), and no one really blames you. The only case you get real blaming is when you don't cc. Mid is 'everyone vs everyone', and no one will notice you're doing less than you can.
**Jungle**, on the other hand, in way more toxic than mid and top. You know. Whatever lane loses, it will be a jungler's fault. Everyone demands an immediate gank not even trying to look at minimap and see where you are. Not even talking about helping a friendly jungler to counter an invasion after 3:00.
**As for ADC and support...**
You know what? Once laning phase is over, jungler's 'sins' are forgotten and forgiven. Unless you're 1/10, of course, but that would be true for any lane. All the blame is shifted towards ADC. Even a jungler can shout "zero damage!" Once teamfights really starts, everyone will blame AD-carry for anything that happens on the battlefield. Even a support can say his ADC is bad. While ADC performance depends oh so much on his laning phase. What can an underfed Caitlyn say? "Blame my supp?" Haha! look your kda/cs, learn to farm, get some dommage, don't die, follow my dash/jump/tp faster.
And now we have a support role. Who can do so much in early game! Ward, catch, protect, zone, watch drake, ss, gank mid, protect turret solo... And who takes the blame from ADC only. Who will ever notice that you've got way too many CS? No one. And in case your ADC tries to talk you may answer he is bad and can't farm (WHOOOSH! Brand's fire column incinerates half the wave). No one really cares anyway. Wards? No one but ADC cares, again. Bad catches? No one will notice. Bad shielding? Doesn't matter. You get caught and die? Shout at your ADC about focus and no damage. You feed 3 kills? But your adc is 0/0/0 and must be okay! Why is he so weak?
Now look at this. Botlane is the only position where you really depend on the other guy so much. And you do not only depend on him, but you get all the blame for his mistakes. While that other guy seldom takes his role seriously and tends to blame you whatever happens. Just like "Klein Zaches, genannt Zinnober". And while you can't really blame anyone after the laning phase is over. What do you think, does it make ADC's life less toxic? I don't think so.
And what do you think? Which role is the most toxic overall?