Learned to fly on Sycamores, not a great type as strange manual control system and badly co-ordinated throttle. Instructed on Whirlwinds, probably the best training helicopter ever, but only one left flying now. also instructed on B47 - easy but slow reactions to control inputs actually require quite a lot of anticipation from student. Hiller 12 - again strange control system - feels like you are connected to the controls by a bungee. Bell 206 - too easy. You can always tell someone who has done their training on a 206, likewise the R44 now that it has powered controls
. R22 - always contentious but here is my view. Students don't have any greater difficulty learning on it than anything else because it instantly shows errors of handling technique, unlike the B47. Unfortunately, the instructor can never relax in an R22, as things can go wrong so quickly close to the ground, and for this reason it is not an ideal trainer (nothing is, as none of the current types were designed as such).
I think the manual control system on the Enstrom demands too much trimming for comfort. Perhaps the best of the bunch from most aspects apart from cost is the Schweizer 300, the CB model preferably. The only aspect I'm not fond of is the marked nosedown attitude in forward flight, which is uncomfortable if you don't have your feet on the pedals as you tend to slide forwards.
Live long and prosper!