Hello!
The rarity of the rewards depends on how easy would be possible to obtain the items during the event.
That said, I can resume by saying that almost every item could be easily obtained during the mini-events played during the anniversary, except for:
changing backpack, 25 years backpack, rainbow torch and lucky dragon kit.
Between those 4 items, changing backpack is the only one you can still obtain nowadays, though is a rare loot from Lord Retro, so let's put it on the 4th place.
Between the top 3, the 25 years backpack was the easiest to obtain, since you only needed to kill Lord Retro one time during the event in order to be elligible to that reward.
And from the most difficult ones, you would need 17,197 points to buy a rainbow torch (you lose immediately 7197 when you ask her for a reward, in order to automatically gets one 25 years backpack) and 57,197 points to buy a lucky dragon kit.
So, in resume, the order would be:
1 - Lucky Dragon Kit
2 - Rainbow Torch
3 - 25 years backpack
4 - Changing backpack