Here are the links to their respective Wiki pages where you can compare the different font faces...
Helvetica
Helvetica - Wikipedia, the free encyclopedia
Century Gothic
Century Gothic - Wikipedia, the free encyclopedia
Calibri (My Fave)
Calibri - Wikipedia, the free encyclopedia
Note that Centry Gothic is much more rounded... all letters with circular parts, including the "O" itself, is a true circle. Most fonts the orbital parts are normally ovals instead. It's one of the differentiating factors with Century Gothic and other fonts. Take a look and tell me what you think. I think Marina is probably right, it's either Arial or Helvetica that is most used in the placards in museums. Both are clean and easy. But the better font, in my humble opinion, would be Calibri, but again, I'm biased.