I love the medals, it's just that I always forget about them. ;n;
I certainly have my favorites, though. Wailord Watcher is still probably my favorite. <3 It's so hard to find that stinkin' Wailord and it's so adorable, so whenever I see it I always run to make sure I get the medal.
I'm actually about to start trying to collect all of the Challenge Medals, which will take a while, but the medal Rally is definitely something to keep up with after you've defeated the Elite Four!