There are lots of eps I like for one reason or another, but that's not the same as being sure that other people would be equally impressed by them (i.e. I love Five, but other people find him bland. Your mileage may vary).

Off the top of my head, though, I'd say that Steven Moffat's "Blink" (Ep. 3x10 of the new series) is definitely one of the best ever. Atypical as WHO episodes go, but sheer genius on the whole.