There might be some episodes I enjoy more than these, but these are the ones I admire for their ingenuity and risk taking. Basically, these are the episodes I couldn't stop thinking about days after first watching them.
Honorable Mentions: Waters of Mars, Listen, Mummy on the Orient Express, Turn Left, and so on. I could probably find something to adore about every episode of Doctor Who. I love that the writers have such creative freedom.
I've only just started the classics so this list might change in the next few months.