The most memorable 18th I have ever played would be The Old Course, strictly because of the setting and all the champions that have been crowned on that green. Second would be Muirfield, because of the great clubhouse behind the green, and all the great champions that have been crowned there (I believe, with Mickelson winning there last year, the last seven Open Championship winners at Muirfield are all hall of famers). I get a kick out of playing courses that have produced so much golf history.
The best 18th hole I have played may very well be Inverness Club. For a short hole, it affords plenty of options from tee to green, and I have made anywhere from a three to a six there.