He did a lot of shitty things, but IMO the biggest was that between him and Thatcher in the UK, they shifted the western world economy towards neoliberalism. The biggest problem with that, amongst many other problems, is that it prioritised short term gains over long term sustainability.

That bit about it not being sustainable is why everything has been getting gradually shittier since about 2010. It's hard to look back on the last 40 years and not see the ideology as being much more than an asset strip of our public sectors and a carte blanche for bankers to fuck about without consequence.