No, I don't have evidence they are 'the biggest'. I'm am overweight middle aged Mum in home counties England with am interest in history, not c suite MI6. However I do accept the opinions of various experts in the area, and a 10 second google would tell you same. Do you? Do you think Iran are a major sponsor of international terrorism or not?
As far as using that Iran are a major sponsor of terrorism as a reason for war, again I don't see the relevance of whether or not its 'major' or 'biggest ever'. I'd argue you are getting bogged down a detail and missing the bigger picture. Iran being a sponsor of terrorism and a source of much destabilization in the ME is seen as a reason enough for war.
As to whether or not these reasons are enough- the prosecutors of this war- in particular the US- are absolutely all over the place when it comes to their justification for this war. We do not have a right to see these reasons- we are lowly civilians. We don't run this country. We elect people to run it on our behalf. These people obviously have access to a lot of intelligence that we don't. You and I don't decide a thing.
I'm not here to defend this war. I was on the fence initially but as it's progressed my feeling has become the following. No matter the justification given, the prosecution of the war, and seeming utter lack of a solid plan for alternative leadership and how to get that leadership in place, makes me believe this war was a flawed venture from the start. I'm deeply concerned the regime will not fall and the Iranian people will be left worse off than before. I believe war always comes at a cost of civilian life, and therefore shouldn't be waged unless the outcome is all but certain. Hence if the regime falls, I'll see it as just. If it doesn't, I'll see it as an egotistical folly on the part of the US ( somewhat different on Israels side but not prepared to go into that here ).
I am still only here to ask you if you actually believe Iran is not a major sponsor of terrorism? And also to push back on your frankly silly suggestion that Hamas is proscribed as a diplomatic gesture.