Rush tickets - which are easiest to get?

Aware that on weekends most rush tickets sell out really quickly on the Today Tix app, but me and my girlfriend don’t have a lot of money to spend on tickets and are in London this weekend. Which shows are in less demand and generally easier to get rush tickets at 10am for on the app? I remember a year or so ago Dirty Dancing always having tickets available most days as did Frozen. I imagine shows like Six are almost impossible as the theatre is quite small.