If you want a strong flavour I recommend Galaxy hops.
I don't know what method you are using to put your kits together. I boil the whole amount of water when I do kits to avoid infection. Infections tend to give you off flavours and are probably the main reason kit beer gets a bad rep.
As it sounds like you are fairly new to brewing, I recommend you google, "John Palmer - How to Brew." Its an online book with charts and things to help you work out how bitter your beer will be when adding a certain quantity of hops at a particular time in the boil.
The more you boil the hops the more bitter will be the beer as a result. Boiling them less retains more of their flavour.