It's always difficult to pinpoint a favorite; I've taken something from every kata I've learned, more from the ones I've repeated and picked apart over and over.
BUT...I always go back to Naihanchi. It just contains so much function in a deceptively, seemingly simple form. Training sanchin as well has always created a nice balance with Naihanchi (or so my experience has been), so the two together certainly form my own core kata training.
Passai, Kushanku, Wanshu...any kata within your path deserves great attention, of course. But when that feeling of training YOUR kata comes, it's fantastic.
It gives any good Okinawan Karate nut the chills!