A lot of subjectivity about what is a success or not, but I would say many nationalised companies (and most were only nationalised for 20-30 years) were absolutely stagnating and/or suffering from widespread union disruption and should have been cut loose. But just picking out a handful of privatisations that went well, I think British Telecom, British Gas & British Airways did much better as privatized companies. Some privatisations went not-so-well - look at steel or coal privatisations or British Rail.

And an example of successful nationalisation - hospitals & doctors were a loose arrangement of private / charitable causes before being nationalised as the NHS. I think we can agree the situation is far better for everyone as a public health service than if it were run for-profit.