You could go for a 290 which is $100 cheaper than a 780 and offers better performance. Out of the three the Asus 780ti will give you the best performance. The MSI 780 would give you slightly lower temps than the Zotac.
You can't really compare cards based on their clock speed. The Zotac might have the highest clock, but it also has the highest temps. The MSI could be overclocked to match the Zotac's speed and would still remain cooler.
Don't get me wrong though, if you don't plan on overclocking the Zotac would be the best value.