I've answered this before, but with time and new collectible releases my answers have changed a bit. Some of these are hard to rank because I love them almost equally, but here's my best shot.
1.
Snowflake Glow Wand - I was one of the billion people who really wanted this to be a collectible a few years ago and I'm so happy it finally exists and I was able to get one. Once the newness wears off, it may drop from my top spot, but I'm sure it will be in my top 10 for a very long time.
2.
Aurora Sky - This collectible has such a unique and beautiful concept. Even though I didn't get my Snowflake Wand during that event, I'm forever grateful to the users who suggested this collectible.
3.
Glam Feather/Cool Feather - I love the feather collectibles and these two are my favorite of all of them. Finding out that they were based on
@Laudine's betta fish makes me love them even more.
4.
White Star Fragment - I love all of the star fragments, but the white one just pops against the dark night-sky background and I love using it in the center of a row to anchor line-ups.
5.
Silver Moon Jellyfish - I am obsessed with underwater creatures, so of course I love our first underwater collectibles. The entire set is lovely but there's something about the silver jellyfish that sets it apart from the rest to me.
6.
Purple Bat Potion - The entire potion set is awesome and I would love to see more added in the future, but the original one is my favorite due to its color and the adorable little bat wings.
7.
Aurora Egg - At one point, this was one of my most coveted collectibles and I will always love it, but it has dropped a bit in my ranking just due to the lighter background not really working in a lot of my favorite line-ups now. As far as eggs go, this is still far and away my favorite egg, though, even with the addition of the Halloweaster Eggs that work better in more of my line-ups.
8.
Dusty Scroll - I'm pretty sure I'll never own one of these beauties, but I would love it if I could. It would go perfectly with the gothic author aesthetic that I love. Maybe add an ink bottle and quill as a new collectible to go with it?
9.
Blue Mote of Flame - Another collectible that I'll never be lucky enough to own, but I think it's gorgeous.
10.
Tetris Grid - Despite some of the other animated collectibles suiting my aesthetic a bit more, Tetris might be my favorite game of all time so I can't help but love the Tetris Grid collectible. Sadly, it's another one that I'll probably never own.