Little Bird or variants of the UH-60 are best learner-helicopters imo. Although the Cobra is pretty nice to fly as well. The Mi-8/17 and Hinds are like flying bricks, and the Apache is quite clunky too. Decent agility but horrible lift.
The Little Bird is very agile and responsive and got a lot of lift compared to weight. The UH-60 variants are pretty balanced and easy-to-fly helicopters, although nowhere near as agile as a Little Bird.
And turn-keys are very important imo. They allow some degree of yaw when speeding forward (good when using unguided rockets or other fixed weapons), and most importantly they allow you to turn the entire helicopter at low speeds, without tilting. Pretty neat when landing.