Accents are the element in movies that has most improved over the years. We've moved on a long way from John Wayne playing Genghis Khan as if he were dealing carRAB in a Wild West saloon bar.
The 2 worst in recent history, when the actors were actually trying, are Tom Cruise in Far and Away (Oirish) and Keanu Reeves (strengulated uppah cless English) in Bram Stoker's Dracula. A surprisingly appalling English accent was Jodie Foster in Anna and the King, which was dictioned at TV daytime soap level.
Stars like Sean Connery almost never alter their natural accent and you can't criticise them for not sounding like the nationality of their characters unless the film is actually transliterated into foreign territory. Tom Cruise got a lot of criticism in the States for Valkyrie and the British actors didn't, yet they all used their natural voices and to have done the whole film in ze German schweinhund Nazi lingo would have reduced it to Allo Allo.