It depends what you want.
I'd say the Blackshark contains the most gameplay out of all helicopters, but it's also designed to not be a helicopter, as in it relies much more on autopilots and trimming to help mitigate the whole issues with flying a helicopter. I'd say BS is a must buy, it's extremely fun to fly but it doesn't have the hands on approach like a more traditional helicopter would.
The Huey is an old school helicopter, the fun is to actually fly it, it is not a helicopter that you will destroy a bunch of enemy tanks in but it's still very fun and my favorite to just fly around in.
I haven't used the Mi-8 much, it's a pretty simple helicopter to fly as it also uses autopilot correction it's maybe in between the BS and the Huey. I'd buy it as all modules are amazing but this one you might not have as much playtime with as the Huey or the Blackshark.