Well, I'm part way through Season 6 in my Friends marathon/DVD run-through.
I love the hour-long episode "The One That Could've Been" which is the "alternate reality" episode. Friends did the "off" episodes like this and the ones that took place in the past very well (even if one of the past episodes fracture contnuity a little bit.

)
Ross never got divorced (the first time!) and doesn't know Carol is a lesbian.
Chandler stayed removed from his job after quitting in season one and is now a freelance, broke, writer.
Joey is a soap opera star on Days of our Lives as Dr. Drake Ramore.
Monica is still fat, a virgin, and dating a boring man.
Rachel never left Barry at the altar and happens upon Ross in the opening segment (bringing her into the fold of the group.)
And Phoebe accepted a job as a day-trader. The episode, mostly, seems to play-out as a "what-if?" pilot for the series and -brilliantly- even has an alternate opening titles sequence.
If I had to pick my favorite episode of the series it'd probably be this one.
My favorite scene: (Ross and Joey are visiting Phoebe at the hospital and waiting for her to return to the room. Ross decides to tell Joey about the
m'enage 'a trois he last night with Carol and Susan (simply thinking it'd spice up his marriage.)
Ross: Hey, Joe, did you ever have a threesome?
Joey:

Look, Ross, I think Carol's great and I'm sure you're a very attractive man but I don't...
Ross: No, the reason I'm asking is... :heh: I sorta had one last night.
Joey: You?! Alright! So... Was it amazing?
Ross: It was... It was... Okay.
Joey:

Just ok.. Did you do it right?
Ross: It's just that... Did you ever go to a party and think, "Would anyone really miss me if I weren't here?"
Joey: Huh... But, Ross, still your worst day with two women? Pretty much better than any other day, right?
Ross: Oh absolutlely. (both share a "guy laugh") It's just that, my part seemed to be over pretty quickly and then there was a lot of waiting around.
Joey: But you got to be with both of them, right?
Ross: Not.. not really. It was just Carol.
Joey: Not the other one?
Ross: No, she kept kicking me away!
Joey: Huh you don't want that. But,hey, at least you got to see a lot of stuff.
Ross: Oh, I saw a LOT of stuff!
Joey: Get a little bored?
Ross: A little. Made a snack.
Joey: What'd you have?
Ross: Just a sandwich. Turkey, little mustard.
Joey: Sounds good.
Ross: It really was!
