At the moment, maybe hard to beat Plymouth which has descended into chaos after some especially vicious internal Tory warfare.
Also trees. Cause it's always trees when the council is collapsing.
Also on the wider point, liverpool are still under oversight from the appointed managers. Heck, the new ward map that's going to be used this year was imposed by them, and since it's basically all SMDs and 2MDs things could get interesting if voters want a change from local labour.
Most people still seem to be expecting a Labour majority, though. This is one place where memories of the LibDem coalition years are likely to last longer than average - and thus a drag on their support.
And, of course, those councils which have little prospect of kicking out the incumbents due to national factors are often the very worst. At the moment that has tended to mean some of the big Labour Mets or London Boroughs (Sunderland, Barnsley, Enfield spring to mind). Liverpool is probably hard to beat at the moment though (and has had issues with pretty shameless corruption, such as Labour councillors having parking fines 'let go').
A similar example on the other side of the aisle would have been Cotswold, though that did eventually catch up with the Tories in 2019 when they lost it to the Lib Dems. Shropshire (not up this year) may be another example on the Conservative side?
Croydon seems to have transitioned seamlessly from being an incredibly unpopular Labour council to an incredibly unpopular Conservative council.