I'm going to shoot my friend's wedding next week and I was told I needed a zoom lens.
What's the best zoom for a GH2?

The best would be the Panasonic G X 12-35mm f/2.8 ASPH ---

Need more info.

The "best" lens is the one in the right focal range with adequate speed to achieve proper exposure in ambient lighting with desireable image quality and size. So there is no single answer. That's why I (and others) have so many damn lenses.

The Panasonic 12-35/2.8 and 35-100/2.8 are good general use lenses with full electronic function.

The Olympus 14-35/2 and 35-100/2 are faster, sharper, larger, and more expensive (unless bought used).

Nikon-mount lenses such as the Tamron 17-50/2.8 and Sigma 50-150/2.8 have more repeatable mechanical focus and parfocal zoom mechanisms, and may be in a more suitable focal range depending upon your placement from the subject.

Super zooms like the Panasonic 14-140 or heck, even the Sigma 50-500 are slow and not generally suitable for indoors, but may be just right if it's an outside wedding and you can't easily move or change lenses.