Any subject which you can get an A in and is not 'blacklisted' by any of the top unis (look at Cambridge or LSE's websites to see 'less acceptable' A-levels and don't choose one of these)! Choosing a language could be a good idea as banking's so international now, but if you're better at another subject then there's no reason why you shouldn't take that, it's your 5th AS and you've already got 4 versatile, good subjects so you can have some flexibility!