Peter Cushing is my favourite actor. He was great as Governor Tarkin, obviously, but I didn't really notice how great an actor he was till I saw his interpretations of Frankenstein and Van Helsing.
He was truly dedicated - he could play any role, no matter how crappily it was written, and make it seem believable. And he just enjoyed acting in whatever people wanted to see him in. By all accounts, he was also a really nice man off-stage. A classic story goes that the one time he got angry with his best friend Christopher Lee was because the latter complained about the food in some film studio canteen!
Tarkin was definitely a fantastic villain. It took a couple of viewings for me to notice that here is a guy who'll kill off the population of an entire planet with no hesitations, seeminly taking delight in this, and dare to raise his voice against Darth Vader. I always thought it was a bit of a shame that he just blew up with the Death Star. Especially since it seems to me that Admiral Piett was intended as a substitute. Having Peter Cushing in the two other OT films would have been great.
That said, Cushing's delivery of his penultimate line is brilliant. "Evacuate?! In our moment of triumph?" That's just so delightfully arrogant!