Which stories have you seen the most often? For me, it is close to a tie between The Pyramids Of Mars and City Of Death.
Probably Pyramids of Mars and The Talons of Weng-Chiang. I had them both on VHS for years before rebuying them on DVD.
But these are the fourteen stories I own on DVD, this is the most comprehensive answer:
14. The Face of Evil (Chris Boucher/Pennant Roberts, 1977) 13. The Sun Makers (Robert Holmes/Pennant Roberts, 1977) 12. Terror of the Zygons (Robert Banks Stewart/Douglas Camfield, 1975) 11. The Masque of Mandragora (Louis Marks/Rodney Bennett, 1976) 10. The Time Warrior (Robert Holmes/Alan Bromly, 1973) 9. The Invasion of Time (Graham Williams and Anthony Read/Gerald Blake, 1978) 8. Horror of Fang Rock (Terrance Dicks/Paddy Russell, 1977) 7. The Seeds of Doom (Robert Banks Stewart/Douglas Camfield, 1976) 6. The Ark in Space (Robert Holmes/Rodney Bennett, 1975) 5. The Robots of Death (Chris Boucher/Michael E. Briant, 1977) 4. Pyramids of Mars (Lewis Greifer and Robert Holmes/Paddy Russell, 1975) 3. The Deadly Assassin (Robert Holmes/David Maloney, 1976) 2. Genesis of the Daleks (Terry Nation/David Maloney, 1975) 1. The Talons of Weng-Chiang (Robert Holmes/David Maloney, 1977)
Ironically, I may have seen some of the lower ranked entries more times than some of the higher ranked stories. That's because I first watched some of lower ranked ones on-line prior to committing to a purchase.