It depends on features you need, as well as it depends on what your own personal preferences are. For me, personally, the best one is the RX300 because I much prefer to use the power curves feature of ArcticFox so, for me, it is not only the best one, but also it is the only good one that exists right now, and that's why I own two of them.
Why do I use power curves? It's because I use power mode, never ever temp control, and, preheat just works a whole lot better with a power curve that lets you fine tune it. You don't need ArcticFox for that, but you do need it if you, like me, hate to futz around with the power curves over and over and over again... because, with ArcticFox, the power output of the power curve scales automatically the moment you change the wattage on the mod up or down, and that's what sets the RX300 apart from any and all other quad battery regulated mods. If going for this mod, then be sure to grab Sony VTC5A batteries (not to be confused with VTC5). Don't use VTC6 batteries in this mod.