I’m gonna split it per sub-collection:
For my FA Supporter/CHR collection, none were hard to get. I’ve basically had a completed collection a month after I started collecting them, with the exception of the Japanese PSA-10 Team Rocket Brief Case promos and PSA-10 The Masked Royal promo, which I bought later on. So I guess The Masked Royal was the hardest to get (just in terms of finances basically) for this subcollection.
For my Seviper TCG collection, it’s defintely the Japanese unlimited edition WCP Seviper. I started collecting Seviper cards, my favorite Pokémon, in 2017, and I was able to complete it in all available TCG languages at the end of 2020 after finally finding this card. The 1st edition version of the WCP set is already pretty rare (because it was only available for online purchase for Japanese players for a two week period, while being a pretty bad set TCG-wise). But it’s nothing compared to the unlimited edition, which could only be bought at the 2008 Spring Battle Road events. This event was supposed to be held at five different locations for a week long period, but due to bomb threads, three of those five locations were cancelled completely, and the two remaining ones only went through their initial two days instead of week before cancelling as well. That, combined with the fact that in this set the non-holo Rares; Holofoil Rares; Ultra Rares; and even the three Eeveelution Gold Star Secret Rares all had the same pull rates, and Seviper isn’t exactly a popular Pokémon to collect or put up for sale, made it a very hard card to find.
For my Moltres WotC #21 promo collection, my favorite artwork, which I started collecting raw, graded, and sealed in all languages in 2017 as well, it’s probably a tie between the sealed Japanese unlimited edition Pokémon Web booster pack containing Moltres and sealed Australian promo. Both took until 2020 to find. Although the sealed Swedish promo, which I’ve only heard about existing near the end of 2020, also took about two years to find to fully complete the sealed portion of my Moltres collection.
For my PSA-10 Ancient Mew sixtet, it’s obviously the corrected version 1, which took little over a year to find after I decided to collect the four PSA-10 Ancient Mew version (the Japanese 2019 and Korean 2020 versions were released later on).
For my Pikachu collection, I honestly don’t know where to even start. There are still 10 Pikachu I’m missing, so those will defintely make this thread once I find them. Especially the German Flug-Pikachu and Surf-Pikachu promos, as well as Spanish Pikachu Volador and Pikachu Surfista promos, which I’ve all four only seen twice each in the past 8-9 years I’m collecting. And of course the Pikachu SNAP promo, not that I’d ever own it…
Some cards from my collection that come to mind, though:
- German Birthday Pikachu, which is still my most expensive bought Pokémon card purchase to this day, and which I’ve also only seen five copies of in the past 8-9 years. These were also rumored to be winner prices for some German tournaments back in the day, so who knows how few copies even exist.
- Japanese unlimited edition E1 Pikachu. The unlimited edition E1 cards only had some very exclusive and elusive releases, like just 50 copies of the unlimited Pokémon-e Starter Deck, and some Pokémon Card GB2 Game Boy Color lottery events where an unlimited edition E1 card was a consolation prize. So although I had been searching for the Japanese unlimited E1 Pikachu pretty actively since I started collecting back in 2015/2016, it wasn’t until mid 2021 that I finally added it to my collection.
- One that didn’t take long to find in terms of years, but was still very hard to find and took quite a lot of searching, was the French SM190 Detective Pikachu promo with movie stamp (and Italian to lesser extend as well). In English and German, this card can be find in abundance for a cheap price, but in French and Italian it’s very rare due to a production error. Because whereas the English and German Blu-Rays/DVDs came with the promo in their respective languages, for the French (including Belgium) and Italian copies of the Blu-Rays/DVDs, the promo inside if was one of the four languages at random. I first managed to find an Italian stamped copy from a French seller. After that I’ve managed to buy one Blu-Ray and four DVDs eventually from French webshops, but unfortunately they only contained a mixture of English, German, and Italian copies. Funny enough, I ended up buying a raw French stamped copy from an Italian seller, 2.5 years after its release of 2019.
- Some other Pikachu cards which took 4+ years to find for various reasons:
- Japanese 2009 Teach Set Pikachu (one of four Teach/Tryout Set Pikachu available)
- Spanish PLATINUM stamped Reverse Holo Majestic Dawn Pikachu, exclusively released in Argentina
- Italian Pokémon Day HGSS Pikachu, which was kinda cursed… It took me about four years before I came across the first copy, which I bought. It never arrived, and I was even too late to get my money back through eBay… The second copy I won for auction never arrived either, although this time I did get my money back. For the third copy (which was almost a year after the first copy I bought, to put it into perspective), I decided to let it ship to a friend of mine instead of me directly, haha. And this one did arrive without a problem.
- Italian Reverse Holo Stormfront Pikachu. Some foreign Reverse Holo cards can just take years to eventually find. Same applies to some other Italian/French/German/Spanish Reverse Holo Pikachu, as well as some Seviper, but this was the last Reverse Holo foreign Pikachu I was missing at the time (with the exception of the earlier mentioned PLATINUM stamped Spanish one).
- Spanish 1st edition Neo Genesis Pikachu
- English Me! Art Academy Pikachu
- English Y2K aftermarket stamped Base Pikachu
- English thin stamp 1st edition Yellow Cheeks Pikachu
- French Shadowless Base Set Jumbo Pikachu
And I could probably go on… I now own over 1800 Pikachu TCG cards, which I’ve started collecting around Autumn 2015, so there have been a lot of Pikachu cards that were hard to get over time. And as mentioned before, there are still 10 more officially released Pikachu left to find, including some more unofficial ones (e.g. the Japanese Glossy Keiji Kinebuchi Pikachu with shifted artwork I’m currently searching for semi-actively).