A Pinky and the Brain Christmas (1995)
Even I didn't expect it to be this good, though.
From a reworked holiday-specific title sequence through to the last joke, this is a great episode. Not quite 100% pitch perfect, but one of the best we've seen yet.
I don't want to give too much away, but Brain's current plan involves conning Santa's elves into making large numbers of hypnotic dolls. There are one or two awkward jokes, but the climax makes up for it. One part made even my Grinchy heart melt a little.
Check this one out if you can. I think you'll be glad you did.
Sadly, WB's been patrolling Youtube, and I could only find short clips online. It's available for a couple bucks on Amazon On Demand, or on Volume One of the DVD release.