I watched Tomb of the Cybermen last night, this would probably be in my top 10, though it does suffer somewhat with datedness. This episode would make a GREAT big budget movie.
Genesis of the Daleks is also a great episode, easily in the top 10!
Honorable mentions: Horror of Fang Rock: This feels like it shouldn't be good..but it is!
I liked Impossible Planet/The Satan Pit, Time of the Angels/Flesh and Stone, The Water of Mars, Silence in the Library, Vincent and the Doctor, 42....
I think from my recent sporadic rewatching of old episodes that I prefer the old Cybermen to the Daleks, and certainly to the new series versions.