My favourites- the Genestealers- Simply for the reason that they prevail in closecombat nomather what you throw at them. For better reckognization of units during games I have them equipped differently with heads and scything talons. Later on I went one step further and gave them distincive color markings.
The Alienstealers were of course inspired by the cultmovie alien(s) and normaly form the bodyguard of my broodlord but since I rarely ever field him they often end up in the troops section. They were converted from oldschool genestealers from the 90s. I used greenstuff for the head and extended their tails with wire covered in greenstuff.