What topics should one master if one wants to pursue a Master's degree in mathematics at a top university?
The simple answer is that if you're interested in a subject and expect to continue on to a master's or PhD (or even if you're not expecting to go for an advanced degree, you simply expect to pursue it as a career), you should take as many classes in that subject as you can.  Pick the best or most interesting to you from whatever is offered by your department to fill out your electives and do your best.