Surprisingly, considering that when I first joined LJ I thought I would have no use for communities ever - and I'm still not too big on posting in them - I have quite a few that bring me regular entertainment.  And because I certainly cannot choose just one, I think we should have a Top Five:

fanficrants : With near-constant updates and several years of old posts, this is always good for near-endless sources of hilarity.
ihasatardis : Level of entertainment described above, times twenty for less negativity. 
office_meta : When it's, you know, not dead, this is the greatest concept ever invented.
polloftheday : Glee!  POLLAGE.  Random polls about everything.
whatwasthatbook : One of the few communities I actually include on my friends page, both because I can frequently help with the children's/YA books, and because it's an amazing resource - it's only failed me once in the year or so I've been using it.

I could probably come up with 15 more (oh crap, m15m !), but then I'd just feel worse about the remaining ones I didn't have room for, so first 5 6 that came to my head it will be.
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P.S. In a completely unrelated note, OMG FLUFFBALL PUPPY.  And now I'm going to...resume writing this cover letter.  Yes.  Because that's fun.  SOMETIMES THERE ARE DAYS WHEN I WISH I WAS BACK IN CAPSTONE.