The problem with this is, how can one compare one horrible event with another, and say one is worse than other? By what measure? Is one war more bloody? Are there more deaths? Does it involve more of the earth's territories and nations? Does it last longer? It really matters what Jesus means by saying this is the "worst" tribulation, right?
Well, I've studied this out, and the best I can do is look specifically at the context of what Jesus was saying. If we compare all 3 synoptic Gospels of the Olivet Discourse, and find them all saying the same things in slightly different words, then let's look at Luke 21. There, this "great tribulation," or "great distress," is being described as a "Jewish punishment." And Jesus said it begins with the destruction of the temple, which was in 70 AD, and lasts until the end of the age.
This can be nothing other than the Jewish Diaspora of the NT age! It's amazing to me that so many are missing this. It is so transparent and clear! Jesus is here, still under the Law of Moses, still talking to Israel as the only "chosen people," telling them that their religion would be destroyed in their generation, their temple ruined, and their relationship with God destroyed until a much later time, following a great dispersion.
Seeing and accepting this plain reality we can see what Jesus meant by the worst tragedy in Israel's history, the "great tribulation." It was the longest period of judgment in Israel's history, and included the complete destruction of Judaism. Their covenant under the Law ended forever, and can only be salvaged under a new covenant, apart from the Law. That is what the greatest tribulation means. That is what the "Great Tribulation" refers to.
It's incredible that this simple reality is rejected, but it is--often! I think it's because people want to believe other things. They might want to see Israel done for all time, replaced by the Church. They may want to believe Antichrist will be the most awful thing in history, from which the Church can escape by a Pretrib Rapture. They may simply choose to see everything through the lens of futurism, and ignore historically-fulfilled prophecies. But it's something I fight against every day.