The Third Doctor - Frontier In Space

They're both excellent stories - some find that Planet of the Daleks rambles a bit, but I like it.  They were the first genuinely linked successive stories (as opposed to finishing one story with the first few seconds of the next) in that Planet of the Daleks actually deals with the situation raised in Frontier in Space, though it has a different location and set of characters.  It also makes some references to the second ever DW story, The Daleks, though you should be able to follow it without knowing that.