That’s a tough one. I think that so far, my favourite is the season six finale, Help Me that aired May 17th, 2010. Overall the episode was very different from many of the cookie cutter plots of the series, focusing a lot on House wrestling with his personal demons and coming to terms with his disability. I think the episode worked on a very personal level exploring a very vulnerable House. It is also one of the few occasions where House is powerless to help his patient despite his best efforts. It all makes for unique drama.
On the technical note, the episode also looked quite different from others, based on their use of a new DSLR camera with a shallow depth of field, making it look visually interesting as well.