How do you write a good epilogue? Do you think the best epilogues are ones that are a specific event or ones that are a general summing up?
Tone, Brevity but above all else - Necessity (as in "is it necessary?") - are the hallmarks of a good epilogue.
How to write a good epilogue... this answer is going to be more opinion than fact and will be done without knowing anything about what it is that is being written (as in the story that the epilogue will be a part of).
First, let's cover what an epilogue is - it contains additional information that isn't appropriate or simply doesn't work inside of the main story. Epilogues can be funny (I like the epilogues in comedies that give a short synopsis of where the characters end up), informative (what happens to the family after the main character of the story dies from his terminal cancer), or even as a way to tease the reader with something more (movies do that a lot with showing that the bad guy didn't really die in the final confrontation with the hero).
How to write a good epilogue. First - do you really need one? If the main story has the Western hero heading off into the sunset, we don't need an epilogue to detail how grateful the village was for his help in dealing with the gang of bandits, nor do we need one to follow the hero for a day or two of moseying to find his next adventure, nor for that matter do we need to follow the fallout to the families of the villains who will no longer bring home stolen bread and butter. Bottom line - not all stories need an epilogue and used incorrectly it can take a wonderfully written story with a solid ending and drag it out (sometimes painfully).
Let's say you think you do need one - there is material there to delve that lies outside of the main story but is needed to give a final catharsis/ending. The epilogue should stick with the tone of the main story - a gritty mystery main story should have a gritty epilogue and not a romantic comedy one.
Provide only what is needed to cap things off or tease the reader - if it had been important enough, it'd have been in the main story (or will be in the sequel).
So I think three things make for a good epilogue: Tone, Brevity but above all else - Necessity (as in "is it necessary") - are the hallmarks of a good epilogue.