Unfortunately the choice of helicopters in the freeware world is always going to be very limited.
There are not so many aircraft builders out there, and most of them prefer to model airplanes, not copters.
Those who modeled copters made them for FS9 a long time ago. Eagle Rotorcraft Simulation made a few for FSX though, like two or three. If I remember correctly, a B206 with many variants, another smaller copter and a Bell 222. The models look really nice graphically but don't expect anything more complex than the default copters...
On the payware world the situation is not so much brighter, unfortunately. The amount of realistic helicopter addons is extremely small. We're talking about less than five here, and I might still be too optimistic. In fact, I can only remember about the Bell 206 from Dodosim, which is still considered as the only ONE fully realistic copter for FSX, and the newest version of the Huey made by Milviz.
Cerasim make gorgeous helicopter models, including 212, 412, 222 and some Hawk variant I can't remember. But their level of realism, while being higher than default, is not top notch either.
MP Design Studios is another maker you want to look at. They modeled the Gazelle helicopter with quite a high realism level. It's not perfect, but at least they tried to simulate quite a lot of important flight model features (vortex ring state, transitional lift, etc...) and they have a good graphic level as well.
Out of these, Milviz and Nemeth made a few interesting model but once again their realism level (engine management, systems etc....) is limited.
From what I could read on the forum, if you want serious helicopter modelling, especially for the flight model, then DCS is the god-sim. This is where the most realistic copters are found. Unfortunately, there are not so many, if I remember correctly. And the world of DCS is too small.
XPlane also has some serious helicopter modelling. XP11 now offers some very high graphic level, and their physic engine give very good results for helicopters. The default copter in XP11 is far from perfect though, but not bad, especially when you compare it with default FSX/P3D copters
Once again, payware is what you're looking for, with creators like DreamFoil for example.