Wii Fit Trainer was probably the most. That really goes without saying. However, this game has several characters that really came out of the left field and I would have never predicted (save for the last several character reveals prior to Robin/Lucina).
Villager was surprising and a funny way to first show off SSB4 to us. It threw me for such a loop... "Is this Animal Crossing U already?" It's really not that Animal Crossing doesn't necessarily deserve a character, it's what Sakurai said prior to Brawl that made me believe it just wouldn't ever happen... That Animal Crossing was, being a non-fighting series, best represented through other means.
The next one would be Greninja, because Greninja was such a random Pokemon to choose. I know, it's a starter, and from the newest generation so it's not completely random, but still pretty random and unpredictable. lol Overall, Greninja and Wii Fit Trainer are the strangest choices, IMO.
Then, of course, Robin and Lucina. Robin wouldn't have been such an out there character choice, but considering the leak and past FE representation in the series, it just wasn't something I saw happening at all. However, I'm so glad it did. That leak had pretty much killed my hype. Ironically, Chrom was the one on the list that I was excited about... But I got Lucina, and I wanted Lucina over Chrom. Lucina herself would be the biggest shock, if it weren't for Robin destroying the leak, though. I thought for sure that FE would only get three characters at max (for a time I thought Ike wasn't going to make it lol), and that Chrom would be the newcomer, even before the leak said so. So, Lucina seemed to be pretty much impossible.