The 550 size heli is generally the largest size helicopter folks routinely fly aggressively on a single LiPo battery. The options in this size class include the Synergy E5, Goblin 570, Align T-Rex 550, KDS Agile 5.5, Gaui X5, Blade 550X, Mikado Logo 550, and others. The Synergy E5 is advertised as a true 550 size. Some of the other brands like Align have several editions of their 550 size class which may cause some confusion when choosing compatible replacement parts and/or upgrades.
Some brands sell as combo kits with electronics where you just have to add your own radio and batteries. They seem like a great bargain initially, but the price tag quickly goes up when you account for hard to resist upgrades and improvements.
Although the Synergy E5 benefits from a few aftermarket improvements and bling that are readily available, the stock kit comes with high quality components, and it has optimal head and tail geometry for the more sophisticated FBL units. The manual is fairly complete, and it includes part numbers. Synergy customer service is also superb; you can communicate with real Synergy experts and even Matt himself with whatever questions or concerns you may run into.
So, if you are looking for a high quality and customizable 550 size helicopter experience, you can't go wrong with the Synergy E5.