3. The End of Time
An interesting one here. When it was broadcast it was bigger than any finale before and that's because it wasn't just the finale to a series, it was the finale for an era and expectations were high. And at that time, those expectations were not met. For half of his final episode, David Tennant sits around and sulks and many would have you believe that is simply it. But it isn't. Because while Tennant is moping, John Simm's Master is single-handedly bringing back the Time Lords. He doesn't know what they've become and he genuinely thinks he's doing something good. If the fact that this was Tennant's regeneration story didn't overshadow what else was going on then we'd be allowed to enjoy its other aspects.