As a fansite item collector myself, I think the hardest ones are the ones that are the least given upon contests and also wanted by most of the players. The frequency of contests are also a factor.
For example, with the Fansite Appreciation Day 2025, the hardest to obtain item was "The Epic Wisdom", which obtained more than 10% of the votes within the entire contest, followed by The Gods Twilight Doll with 6.21% and Crunor's Heart with 5.90%.
Another important factor is the contribution system like we have here. Right now I only know that TibiaQA and Guildstats have a contribution system, maybe TibiaDraptor as well? I don't understand their contests, so don't know if they also have one.
Another factor is the prizes that each contest gives. I won 3rd place on PortalTibia's contest and earned an Epaminondas, but all ranks (1st, 2nd and 3rd) wins an Epaminondas, so I think this one is an "easy" fansite item to obtain. Not considering the difficulty of the contests itself, as everyone has the same chance, but because they give the item for all the winners, not only the first like other fansites.
But in my opinion, the Fansite Appreciation Day is close to the chance you have on each fansite item, despite those contribution systems, prizes and also contest frequency factors.
For example, right now the Memory Box from TibiaEvents was voted by only 12 people, so you have a higher chance of getting it.
Then we have The Epic Wisdom from TibiaWiki BR which was voted by 543 people, so if you choose this one, you are competing against 542 other persons, your chance is like winning a lottery.
Even on contests, probably the majority of those people that voted on the item will still participate on new contests depending on difficulty of the particular contest, but it's still hard because you will always have more people wanting the specific item.
Maybe in the future polls of Fansite Appreciation Day, the votes per items will change, as some look at the least voted items and will try them instead of the most wanted ones, but it is a fact that those 3 Items (excluding Crunor's Heart as TibiaBosses left the fansite program) are the most wanted ones because of their effects and sprites.
As for another difficulty example, there is the Baby Munster from Rookie.pl, which is always related to content on Rookgaard, so to participate in their contests I advise you to create a character on Rookgaard and they also keep the item winners on their page, so it's a bit rare to find one, since they give him only to a few people.