The current WWE World Heavyweight Champion Seth Rollins won his first World title back in WrestleMania 31. He cashed in his Money in the Bank contract in this event that shocked the entire world. This cash in is still considered as the best ever Money in the Bank cash in.
Nowadays, Money in the Bank had mostly been a filler event and over the years it had been totally irrelevant. Since 2017, Money in the Bank did not make any difference whatsoever. The last time Money in the Bank seemed a little relevant was back in 2016 when Dean Ambrose won and cashed it in on the same night to win his first and only World title.

Seth Rollins Opens Up About The Creative Process Behind His Memorable WrestleMania Moment
But after 2016, none of the Money in the Bank winners had any important role. You may argue that wrestlers like Brock Lesnar and Braun Strowman won it but honestly, they did not need it. Money in the Bank was designed to put over a new talent. It was used in this exact way when it started.
But with time it became completely irrelevant. For the last number of years, it had been no impact at all. Nowadays, fans are not bothered about Money in the Bank at all. LA Knight is rumoured to win Money in the Bank this year, but we can pretty much guarantee that he can never be a top star of WWE. He can be an upper mid card at best.

But the cash in of Seth Rollins is considered as the best Money in the Bank cash in ever. He was being built as the top heel of the promotion during the time, and at WrestleMania 31, Roman Reigns and Brock Lesnar fought for the WWE title. Rollins shocked the entire world when he cashed in his contract on the main event of WrestleMania and to win his first World title.
Seth Rollins recently spoke on the “Mark Andrews: My Love Letter to Wrestling” podcast where he discussed this epic Money in the Bank cash in and explained how it came through. He also revealed if there were any other plans about his cash in. Here is what he had to say;

“I had the idea to do the cash-in a couple of months before WrestleMania,” Rollins said. “So probably January [2015], December [2014]. I’m riding in the car with Cesaro, Claudio Castagnoli, and we’re booking the territory as we do, and I’ve got these ideas and I’m running them by him. He’s got his ideas, and we’re just trying to come up with some stuff, and it seemed like a good idea, but I had to find the right time to put the bug in somebody’s ear.”
Credit “Mark Andrews: My Love Letter to Wrestling”
H/T to Wrestling Inc. for the transcription.