Declines suck but I guess I am happy in a way for each one because (sorry @muppetoverlord) no shows are even worse.
Most of mine came in the traditional decline checked on the RSVP card kind of way, but a few were a little more thoughtful and softened the decline sting. Some of these declines that came back with nice little notes. These made me think that if I have to decline in the future there could be more thoughtful ways to do it.
The best 3 declines were:
1. The friend that thought she would be able to come but then had a business trip scheduled across the globe. She called me in advance of the RSVP date, explained the situation and then we had a nice conversation. Who doesn't like getting an actual phone call from a friend? 
2. My dad's OOT cousin that H has never met and I only see on rare occasions. He wrote a note saying he was sorry he couldn't make it but he really enjoyed looking at our wedding website. We put a lot of effort into our website and it made me happy to have someone mention it like that. It is also good to know that when H does meet cousin he will already have at least some familiarity with H from some of the website info/pictures.
3. My 2 hour away friend + guest that I feared would be a no show sent me a text the day before the wedding saying she had to work and couldn't make it. In one way it was the worst because it was really last minute but since it saved us from having any no shows it ended up being a good thing in my opinion.
What were some of your best declines?