Great topic! I'm a college teacher, and I actually use this question on the first day of class as an icebreaker (the non-baseball folks get to tell me what they consider to be an 'essential album, painting, or book' or their favorite thing to eat, but I digress.)
Current favorite: Zito. Absolutely blew me away last year when I watched him take a no-hitter into the 8th against the Mariners. It's the closest I've been to seeing a no-hitter, and it was the most riveting sporting feat I've ever seen.
All-time favorite: Dave Stewart. Nobody stared down a pitcher like he did. Except maybe Bob Gibson, but that was before my time.
Honorable mention: Satchel Paige. I mean he was, what, 49 when he played for the Indians? That's just cool.