I'm sure I'm not the only one that secretely LIKES diacetyl...
I LOVE a nice buttery diacetyly Chardonnay (Bill Blass's Presidential Chardonnay at O'Brien's is a great, 2-sticks-of-butter flavor), and a nice buttery beer every once in a while.
40 Foot in Dun Laoghaire (yea, I know...) has Pilsner Urquell on tap and at least the current keg, is SUPER buttery. If you are looking for a really buttery beer with loads of diacetyl to try and identify the flavor, the Pilsner Urquell on tap at 40 Foot at the moment has it in spades.
(Maybe I just didn't expect it but I've never noticed diacetyl in bottled Pilsner Urquell before.)
Adam