Same the others, really: DMP because it was one of the biggest stories the original series ever told, and the surviving episodes are intriguing. Plus we had an early appearance by Nicholas Courtney, plus Jean Marsh in leather -- need I say more?
The Tenth Planet because not only did we get to see the Cybermen for the first time, but also the first regeneration. I'm told the first 3 episodes were issued on VHS in North America years ago, but I've never come across it, so the whole story is new to me.
Power of the Daleks, because I want to see how they handled the first changeover in Doctors.
The Invasion doesn't really count here, though, because it has been completed thanks to the animation. Yeah, of course if they ever find the two missing episodes I'll be happy, but I don't really consider it an incomplete story anymore, especially as odds are we'll have to rely on such reconstruction if we ever want to see these older stories. I think there's a great opportunity for some talented animators to go to town on Marco Polo, for example (which would be my fourth option if there was one, because not only was it the first great historical, it was reportedly quite spectacular for the time, and it also would fill a gap in the very earliest era of the series).
Alex