My favourites are two campaigns that actualy were in between campaigns, a couple of sessions to blow off some steam before continueing the main campaign.
I played in a Dark Matter'ish campaign (using a simplified Top Secret rules set) that lasted 2 sessions and was based in the Profecy films (we were a special crime unit investigating angels ripping each others heart out and stuff)
I also DM'ed a campaign based on the Profecy films based on the scenario I played in but with some different plot twists. I used a simplified and adapted rules-set based on the L5R system and ran it as a X-files episode (they even got an encounter with Mulder

) It took 3 sessions.
I liked both because...
They were something completely different (not the regular system and definetly not fantasy)
They allowed for some real roleplaying since both required investigating rather than fighting. ( in the Dark Matter'ish campaign I remember we had to avoid combat at all cost, or our little crime unit of 5 detectives would have had to deal with a full blown gang war. Some of the players thought they could out-play the DM and got a bullet as a result. You've got to love a good dfive by shooting, at least if your not on the receiving end)
They were very intense, the pace was very high, like a TV-show episode and there was no healing: a bullit in the head...
Other than that, from those I played/DM'ed I prefer Greyhawk and from those I never played: I definetly would like to play in a Celtic flavored campaign (something like Harnmaster or Slaine)