A gun drill would be ideal, but very expensive, especially since you will almost certainly need to buy the coolant unit to go with it.
If the metal is too hard for HSS, then you will want an insert drillbit -search ebay for carbide spade drill, you will see the sorts of inserts on offer. You would need to buy or make a suitable holder; the former is likely to cost a few quid, the latter less so, but you need to worry about accuracy...
Is the metal hard all the way through? I've never tried machining a hydraulic cylinder; I do have some pins which would have been used in bosses at either end of a hydraulic ram, and they are as hard as Vinnie Jones on a particularly hard day - I've burnt a couple of carbide tips out trying to shave them to size; they do, however, tend to be softer in the middle, such that an ordinary HSS drill bit works just fine. Until you hit the hard layer, that is, then it all just goes to hell very quickly...