I contacted Zeiss last year about weather sealing and they confirmed that there is none, unless you buy the more expensive, bespoke, industrial series. Having said that, the same lack of weather sealing applies to my medium format Mamiya lenses and my large format Cooke, Nikon, Schneider & Rodenstock lenses, all of which have been used in light drizzle.
Zeiss glass for both medium and 35mm format is excellent but again choosing it is also a personal matter depending upon what you seek to obtain from a specific lens. I also rate certain Mamiya lenses very highly. With the Zeiss ZF series I would avoid the 25mm but think the 21mm and 35mm f2 are superb lenses. The latter is certainly an improvement on the excellent Nikkor 35mm f1.4, and around the same price.