You (try to) never operate a component at its limits to reduce the risk of a component failure. For electrolytic capacitors as a rule of thumb you use a rated voltage between approx. 50% (or more) higher than the expected max. operating voltage.
With nominal 16V operating voltage you'l have to take into account overvolatges from mains of 10% to 15% which will rise the voltage to 18.4V. Add 50% headroom and you are at 27.6V. Typical rated voltages for electrolytics within this voltage range are 25V and 35V. 25V is lower than the calculated value, therefore 35V is a good choice.
Of course, in your application a 16V capacitor may suffice, especially if you provide additional overvoltage protection (e.g. a 12V zener diode) as is advisable for a bike dynamo anyway, as the output voltage without load may rise considerably above the typical 6V you might expect.
Here's interesting stuff on this topic.