This page is a good explainer of IRV's weaknesses and shows some of the alternatives: https://ncase.me/ballot/
Since you mention it, let me offer a candidate for "worst possible voting method (that has actually been used in a democracy)", namely "plurality-At-Large".[0]
It basically takes the worst properties of FPTP and extends them to an entire governing body, allowing a party with less than 50% support to win 100% of the seats (by design).
Fortunately it has been mostly abandoned in the US thanks to the Voting Rights Act and subsequent SCOTUS decisions.
[0] https://www.nonprofitvote.org/the-bias-of-at-large-elections...
A close variant (vote for up to N, top N win, at large) is used in a lot of local elections still; all multimember district methods are banned in House elections specifically to prevent plurality at large from being used there. (Though the Senate uses temporally-segregated two-seat plurality at large in every state.)
Correction: it uses either that or similarly segregated majority/runoff, varying by state.
Majority/runoff and plurality are similar FPTP variants, but majority/runoff is slightly less bad.