For me it was the Marie BJ in the Christmas special, for numerous reasons. First off I suck at maze games, especially without a mini map, so having to replay Krampus Fexa's maze every run was not fun, and there's this one I still have yet to properly pin down its location and I always find it by only chance while I'm running around aimlessly. Second was when I originally played part 2 I couldn't even find enough coins to get the basic radar, but since I saw MasterMax888 managed to keep his radar when he started a new run, at first I thought maybe you can keep the coins you got from a previous run, only to find out that wasn't the case, so I was under the belief you needed to play it on higher difficulties, (I only play this on easy mode since I suck at this game) so for a few months I didn't even try to play that mode again and try again, then when I replayed part 2 I learned I got the snowball gun right away, later in the run I discovered the blue coin bags, which ultimately gave me enough for the radar, enhanced radar and the mini scene where Marie shows off. Then on my third run I made sure to collect every coin bag I could in part 1, and when I got to part 2 I made sure to check everywhere I could in each area before leaving said area to assure I didn't miss anything, and in the end I barley had enough for the scene. So ultimately I'd say that was the hardest one for me since I had to play the entirety of the Christmas special 3 times and search for coins which ultimately took me about 4 hours.