What green tea has the highest caffeine content

First off, the ISO standard is not intended to produce a good cup of tea. It is designed to produce a consistent one for taste testing, so that no tea manufacturer can claim that his tea wasn't made "properly". It's title is "Tea -- Preparation of liquor for use in sensory tests"

As for the actual tea making, yes, leaving the bag in longer will make a stronger cup of tea. The concentration of caffeine (along with flavour molecules and everything else) will slowly trend towards an equal concentration in the leaf and in the water. The longer you leave the tea bag/leaves in the water, the closer to equilibrium you will get.

There are other factors that affect this, such as the temperature of the water, cut of the leaves, bag versus loose leaf and so on, but the trend is always towards equilibrium as time progresses.

I'm not sure where the upper limit of this lies, but I think that once the cup is cold there's no point in it anyway. Thus, leaving the bag in for an hour is a bit much. I usually steep my tea for 3-6 minutes, depending on how strong I want it to be.

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.)