He was *at least* the second most powerful man in Iran. The argument could even have been made that he was the most influential. Khamenei almost always acted on his recommendation. He was the most powerful person in the IRGC, even more powerful than the IRGC chief. The IRGC is by far the most powerful entity/organisation in Iran. By most estimates, it controls over a third of the country’s economy and essentially all of its military power. There is no analogue for it in the West.
To give you an idea, you will find stories of Soleimani giving ultimatums to successive Iranian presidents. He was the single most important person in defeating IS and has for decades been a major thorn in America’s hegemonic ambitions for the Middle East. The New Yorker described him as the most powerful operative in the Middle East.
I have seen his murder being described as equal in significance to Iran assassinating the VP. The reality is that it’s much more significant than that. Aside from Cheney, the VP has historically wielded very little power or influence. It’s almost impossible to understate the significance of this murder and a forceful Iranian response is guaranteed. It’s not a question of whether Americans will die because of this incredibly foolish and reckless action, it’s a question of how many and their seniority/importance.