This is a hard question to answer even for a hardcore sticker collector. There's so much information we don't have about certain sets, some sets we don't even know where they came from or how they were distributed.
Another question we should be asking ourselves is : What do we mean by rarity ? Does rarety = high prices ? Is the physical condition of the sticker taken into account in the rarety assessement ?
Below you'll find a tier list with a lot of sticker sets listed by rarety and the explanations to my ranking.
Which is the hardest to complete / rarest sticker set ?
Below are the criteria taking into account for the ranking of the sets by rarity / difficulty to complete. The rarity of sealed products are not taken into account. The ranking mostly applies to stickers bought directly from the source in Japan.
Scarce stickers due to low production
Rare design variants
Low quantity distribution (the sticker only came as singles or in pairs)
Rarely appear on the market.
Rarely appear on market
HUGE set with no available lists on the web
Always some available online
Some parts of the sets came be hard to find / expensive
- Huge sets
Relatevely easy to find and always available on the market
Not very expensive
Polluting the market
Small Set (ensky 2006)
Why aren't prices taken much into account regarding rarity ?
The rarest stickers of all time can appear randomly for cheap, so it's more a matter of "how often do they appear ?" than "at what price do they appear ?"
Value for stickers (applies to any niche collectable) is pretty complex, too many factors have to be taken into account, so there isn't a fixed value. A lot of people would spend a lot more on a single sticker that completes a set than to start a set. Some people have tight spending budgets or like hunting for good deals and some people don't look at what they're spending and some people resell stuff to afford collecting. Finding a single sticker is way harder than buying bundles.
It also needs to be pointed out that most of time when a person buys a sticker from Ebay at a high price, even though the sticker is from the Amada 1997, it's because usually they don't know about the set, nor do they try to find the sticker at a better price. It's just so easy to buy from the most visible platform.
There's no correct way to collect stickers. You do you ! Collect half a set or only the holos if you please. As long as you like what you're collecting, that is what's important in the end. Start by collecting the set that attracts you the most.
One last thing : trying to complete sets without getting any duplicates makes the sets much much harder and expensive to complete.