Wolverine SMP
The Wolverine SMP or "Single Moving Part" is one of the newer systems on the market and our preferred system here at HPA. It is a simple drop in system that replaces the internals in your current gear box. Created by Rich Lort, it's simplicity is why we love it so much. Because it utilizes a single moving part the Wolverine SMP is extremely reliable and the maintainance is a breeze. It's easy to program and it's performance is astoundingly consistent. The last reason this is our favorite system is because the retail price makes it the most affordable system on the market saving you over $100 as compared to the reigning king of HPA...Polarstar.
The PolarStar is really the grand daddy of Airsoft HPA systems. This tried and true system has been the go to HPA engine for the last several years. It is slightly more programmable than the SMP and is a complete drop in gearbox.
The reason why this is our second choice is because it doesn't necessarily fit in any gun shell whereas the SMP fits into your existing gearbox. That being said this is a phenominal system and one you really don't need to worry about from a reliability standpoint. However, that comes at a cost. At a price of $500, the fusion engine is the most expensive HPA System out there, but you do get a lot of bang for that buck!. The last reason this is our number two is because it has a lot more moving parts that can go bad, but it's so well made it's rarely an issue, but believe me, it's not as easy for the DIY'er to get into and do maintainence with this unit.
PolarStar Fusion Engine
There are a couple other systems, but in our experience...these are the best options. The customer service and support behind Wolverine is top notch and Polarstar is tried and true so you can't go wrong with either of these options. Word on the street is that there is a drop in unit that Polarstar is releasing soon called the F1 so keep an eye on that as well.