It's too hard for me to actually vote in the poll because of the medals always changing, so there's no specific weekend that I think always has the best medals. But in terms of the medals I've earned, my favorite is the Dark Side Challenge Death Star one from this year. I love it so much. The 2015 Wine and Dine medal is also great, but my feelings towards it as a whole are a bit tainted since it wasn't really a half marathon.
One of my favorite medals overall that I don't actually have and am sad to have never gotten the chance to earn is the Princess Half medal with the heart and tiara. I think that one is so pretty and princessy and much better than the enchanted rose one they used this year for the Belle theme.
As others have said, Marathon Weekend's medals are also usually pretty cool with the classic characters. They better come up with something really special for the 25th Marathon next year.