Originally my favourite had always been werewolf and probably always will be my true favourite. You can be really sneaky using the pounce ability to parkour or reach spots that humans wouldn't expect an infected to be able to get to, makes you feel like an assassin when you jump down onto unexpected humans haha.
However, since getting zpower 4, I've been playing normal zombie over an other kit (unless situational), purely because the armour and the weapon that it has in my eyes is probably one of the best (even without zpower). With the lack of abilities that zombie has, zpower definitely makes up for it. That's the main reason i use zombie, other than the fact humans might not expect a regular zombie to do as much damage as I or anyone else with zpower 4 (because typically you see a rego zombie and you think quick easy kill).