Nuclear or hydal power which one is best & why ?

Both have pros and cons. Both are good if planned, operated ans regulated properly. The most important thing is  a country must come up with energy mix policy that may include both considering its natural resources, geopolitics, etc.

hydel is best no comparison of hydal with any other power generating source, hydel running cost is very low and more important hydal don't have any risk to our society, however nuclear power palnt have great potential of radiation release which can very dangerous for any society, you can see recent event at FUKUSHIMA Japan. 

hydal is best because it is cheap, easy, less harmful