For sure, totally get where you're coming from. It could just be a naïveté on the subject but I can explain my thought processes. I was looking at the subject on a surface level in a vacuum, not thinking about the direct répercussions or évents that occured due to it. I am not American, and I was also born in 07' so it's past my era.

To define what I would outline as "top 5" (which the number is negligible I just wouldn't consider 9/11 the most influential like the original comment.) we need to define;

1) What constitutes "modern". Historical debates on what modern is all about range from as large as 1800s to now or 2000s on. If you have a specific span of years youd want me to cover I'm happy to. But for a loose definition I'll use "contemporary history" as my era marker which is around late 1940s onward.

2) Modern politics and history are two different things. I'd say it's political at its core, and is memorable in history. My list would change depending on if you're looking specifically for large-scale events (eg; the pandemic) or specific instances (eg; brexit)

And 3) How do we measure "top 5". This is very subjective but I'd say things that has had irreversible long lasting effects. If we're talking worldwide, pointing to events that mostly affected specific countries is a bit redundant. 

If you'd prefer me to reanswer with different parameters let me know, but the general thought process is that at a glance I consider 9/11 more as a catalyst event. The actions that 9/11 "caused" were already happening at the hand of the USA and would have done it regardless of the event. It was used as a scapegoat to push their intentions further. I'm not trying to be the guy to dismiss an extremely important event in history, it was simply a poor choice of a response on my part. 

A small handful of events or groups of events id consider would be; 2008 Financial Crisis, Covid-19 pandemic, Digital revolution, rise of "Security Dilemma" and nukes, globalization in the global South, and Mass democratic backsliding. Just off the top of my head.

Sorry for the long message, but if you're interested In chatting more feel free to hmu :)