Which Type of Japanese Green Tea Has the Highest Amount of Caffeine?
From what I've read, caffeine content can be variable even within varieties. There are possibly seasonal variations, variations in harvesting and processing technique, variations in the original plants... Psychological effects (e.g. what smells perky to you) might be just as strong, and steeping for 10 minutes instead of 5 might make as much difference as any choice of variant. So take any answer with a grain of salt, and don't be afraid to simply go with what seems to subjectively work best for you.

That said, gyokuro is supposed to be stronger than other sencha teas, and I believe compared to other Japanese green teas as well. (I found similar information elsewhere; it's not just wikipedia.)