Quite late to this, but seeing noone speak in favor of the hind hit me in a sensitive spot...
The Mi24 is not nice. It won't forgive mistakes and you gotta play by its rules, but once you learn how to fly it, all the other heli's will feel too easy.
It's heavy, restrictive, but playing with that will really fine tune your skills. Despite all of that, it's the fastest heli out there, and it can do everything. With VERY capable weaponry, decent range, tough armor and the capability to transport troops and cargo (useful for dynamic campaigns) it's the choice I recommend. The F18 of helis, if you will- though maybe a little more rustic. Hey- It even has autopilot* !
It might not be the best choice for a starter, it will be frustrating but personally, whenever I'm not sure what mission I'm going to fly, I pick the Hind and it always gets the job done. You can even kill f14's with that thing (tried and tested!) and the Petrovitch AI is competent enough.
In the long term, it's the one you'll love the most, and thanks to its well designed trim system and stability assists you can let it fly itself at max speed for a while, you don't need to baby it (like the Apache) if you're trying to reach max velocity.
It ain't the easiest heli, but the most fun and challenging one, according to myself. So maybe not the best to learn... but if you were to get only one heli, I'd recommend it, still. (To be honest they're all pretty great, quite unique... Huey textures are a bit old as of today, Gazelle as well... but they all hold up)
*Autopilot, but it's kind of more like a half assed auto-hover mode that only engages when it feels like it... Hey, at least you'll spend more time flying the thing!