No "Yesterday's Enterprise
I very much love "The Wish". In fact, I started paying attention to Buffy
in early S3, and "The Wish" may have been the episode that cemented my interest in the show. The slo-mo sequence at the end is one of the finest sequences of the series IMO.
It's a shame Firefly
never had a chance to do one.
While "Parallels" probably doesn't quite count, it remains one of my favorite episodes of TNG.
"Perfect Circles" from Six Feet Under
has a sequence where one of the main characters jumps through several alternate timelines, and even after things settle down the show has jumped forward in time enough that for awhile you feel like you're still watching one of them.
SG-1 had a number of alternate timeline episodes and even a movie that may merit consideration as well.