Definitely the 12-35mm f2.8
The Mitakon Speedmaster 25mm f0.95 is both a whole lot cheaper (found for $369 on Adorama) and with tested better image quality than the Voigtlander alternative, and for a Micro Four Thirds lens with a brilliantly fast aperture, it's (in my opinion) the best choice, but I find myself still using the Panasonic 12-35mm f2.8 OIS lens more often. It's just too versatile.
In my opinion, the absolute best camera you can get in within that price range is the Panasonic DMC G7. It's like a Panasonic GH4 without some of the extra features (such as slo-mo video) but it shoots a brilliant and cinematic 4K video image and captures great still photos as well.
Here is an example of it's video footage:https://www.youtube.com/watch?v=FUxDvGD2Ask
You can find it on eBay for $436, but you will also need a lens. If you insist on a very inexpensive zoom lens, you can find the Olympus M. Zuiko ED 14-42 f3.5 - 5.6 Micro Four Thirds zoom lens (currently found on eBay for as low as 89 dollars.)
The total cost could be slightly over $525, but well worth the extra money as nothing else in that price range can capture image quality this good. The Panasonic DMC G7 is also an interchangeable lens camera, for more versatility in the future if you decide to invest in different lenses. With an inexpensive lens adapter, it can also take Canon and Nikon lenses.
These both sound amazing... especially the 14mm
That's really freakin cool...
Sigma Art 18-35 f1.8 and Sigma Art 50-100mm f1.8 is a great combo if you are able to spend $1,900 (The 50-100mm F1.8 does not have IS, however.)
A brilliant combo for short films etc..