That depends on you current and future setups. The best common product that will allow you to make more adjustments is the S.E.R.T, next I would go with the PCIII and then V&H power pack.
My 2 cents in order. I have found and done research and still researching is the thundermax ECM which you can make lots of adjustments but also auto tunes you bike with the parts you have and as your bike drives in which the others don't. With the others to get the best results you should dyno even though you don't have to in which the thundermax you don't.
Many options to choose. Read other posts and research.