I'll just leave this link here so you can look up the rarest mounts, minions, titles and achievements within the game: https://lalachievements.com/en/rarity/title/global/

In regards to Event Items: Either you've got them during the event or you didnt. If you didnt, you might be able to buy them on the mogstation (thus, they're really not "rare" or any kind of achievement) or you cant (in which case you cant really aim to get them - you can only wait for them to be added to the mogstation... which is going to make them less rare again).

In regards to Glamours... what do you even mean by that? Single pieces that are difficult to obtain? New combinations no ones daring to try?

In general my personal advice to you would be to set your own goals and dont base them purely on rarity or even the advice of strangers (myself included here). Some of the rarest items might involve content that might not be enjoyable to you - Eureka, PvP, Bluemage, Ultimate Raids, Solo PotD, Hardcore-Endgame Crafting, Hunts...
Personally I'd make something I enjoy my "endgame" focus and after that look up goals that I can achieve within this content of my choice - for example: I enjoy crafting, so getting this Pteradon is a good goal for myself. Not only, or even mainly, because its rare but because its something I can achieve within the content I enjoy. On the other hand I strongly dislike anything Bluemage-related - so I'll probably never get the Morbol. But even though thats a rare mount and would be perfect for "showing" off, I'm not prepared to do something I dont enjoy.

So... figure out what you enjoy, see what kind of goals you can set for yourself and go for it?
And if its really only rarity you seek: Give that website a try, thats gonna give you a good idea what you should aim for if you mainly want to get your hands on something other people dont have.