Tooms (counted as one) - Best... episodes... ever.
The mother episode, if you've seen it you'll know the one.
The one with the deformed mother under the bed
The carnival town episode is one of my favourites from series 2. Very good horror serial, worked really well and appealed to my sense of taste
I loved the episode with the green bugs in the first series too, that worked very well.
The episode in series 6 with the navy noise test and Hal from Malcolm in the Middle is really awesome. That's definitely one of the best episodes, the car chase covered on tv works really well.
Dunno, there's lots of very cool episodes. Haven't got up to series 7 yet to rewatch, but i remember the COPS episode was very good.
In terms of originality, the bermuda triangle episode was unique and fresh (very different camera work, especially with scully storming round FBI trying to get some satellite navigation info), the xmas one with the ghosts was another great original episode.
The alien abduction writer episode was a funny one, lol @ the men in black. "you know venus is very visible this time of year!". Not one of my favourites in anyway, but its a good one