My favourite? I know it sounds strange, but it's the first wave of figures:
Princess Leia Organa
Why? For purely sentimental reasons. Without these steroid freaks we'd never have progressed to the quality we have today.
Now, in terms of quality of sculpt, I'd choose the AT-AT Driver and CT Stormtrooper, as well as Ree Yees, Gamorrean Guard, Admiral Motti, Grand Admiral Thrawn and Grand Moff Tarkin.