It was a tough choice between Psychic, Ghost, and Dark, but I picked Psychic in the end. I always like working with all three of those Pokemon types in the games. But I think Psychic wins because it's been my favourite for the longest time, and I always find myself cheering when one of my Pokemon learns a Psychic attack. And I'm even unwilling to get rid of Psychic on any of my Wormadam, even though in some cases that might not be the best attack for them...
Plus I LOVED how in R/B/Y Psychics were immune to Ghost-type attacks. True, that made Sabrina a lot harder to beat, but... Oh and speaking of Sabrina, she's my favourite gym leader AND anime character, so maybe that's another reason why Psychic's my favourite type.
But like I said, Ghost and Dark are close... after all, my top favourite Pokemon (Mismagius) is Ghost-type, and my third-favourite (Darkrai) is Dark-type. And no, I didn't pick Fire because... well I dunno, that type doesn't compare to Psychic/Ghost/Dark, even though my second-favourite Pokemon (Ninetales, of course) is Fire-type. Plus, quite a few of my favourite attacks are Ghost or Dark-type, such as Curse, Thief, Dark Pulse, Dark Void, Grudge, and Shadow Sneak.
Interestingly enough, during the days of RBYGSC, Fighting was my least favourite type. But then that type grew on me when Medicham and Lucario appeared, and of course Aura Sphere, another of my favourite attacks.