In which she about dies of stunned laughing

Skullcrusher Mountain a Doctor/Master crackvid. The Doctor's expressions are fantastic. And the long moment where the Master pauses in his evil scheme to lovingly study the Doctor's sleeping face. And the... the whole verse about the monkey/pony monsters is... I don't think I'll ever get tired of that bit of spectacularly wonderful WTFery.

My other favourite Master fanvids are both from Ten's era:

Anything You Can Do I Can Do Better - is another Doctor/Master fanvid, just as hilarious, not quite as much "...What did I just see?"

Don't Stop Me Now - a joyride through the Master's life as Saxon.