According to independent speed measuring company, Vodafone has on average the fastest 3G and 4G network in Czech. http://www.dsl.cz/clanek/3237-namere...z-v-lednu-2015
It also has a dedicated foreigners website with offers just for them: http://www.vodafone.cz/visitors/. I can recommend to visit Vodafone's Prague airport store. Right at Terminal 2, a 3-minute walk from Terminal 1. The level of English has also enhanced at many other stores downtown and in other towns. http://www.vodafone.cz/en/about-vodafone/shops/

No experience with other providers. Enjoy Czech!