I have to say, I really liked the recent Battlestar/24 crossover. Sure it got soapy and over the top, but it was almost like an insane "what if?" with fallout I can definitely live with.
I sort of felt like the season premiere was the real series finale (downer), and now everything afterwards is gravy. Like the time travel episodes that made up the post-graduation episodes during the final season of Felicity.
Lost is also on a roll...if you fastword through anything with the Oceanic Six.