Actually, it's slightly different on DMM. Now, we have only 5 kind of event: Raid Boss, Card Flip, Treasure Rally, Event Gacha and Sapling Raising (also known as Nae-chan event) + Poll (but it occurs only once per year for popularity voting).

My favorite event are Sappling Raising and Raid Boss. I like Raid Boss event because it's easy and it allows you to spend your stamina for other stages. And Raid Bosses are a good way to play with Keria ridiculous strenght.
I also like Sappling Raising a lot because Nae-chan and because it remind me the first one with first Nae and Azalea. One of my favorite 2018 event and maybe one of my all time favorite event.

My least favorite event is... Card Flip. It needs a LOT of currency and time to get everything and avoid Nazuna cakes. Hopefully Christmas' one don't have that much Nazuna cakes and Turn all card option helped a lot.

My very VERY favorite kind of event is Nidhogg event but since it's a side event, I can't count it. First Nidhogg event was one of the best thing I did on the game. I hope we will get event like this in 2019.