I'll share my favorites, though I'll leave out ones not released on Nutaku's version so far since I don't really know 'em. Also, my list is only taking into consideration two things: appearance and maybe personality. I don't care about stats/skills/abilities for Event girls since I've never used almost all of them.
Favorites:
1. Eucharis [Base Skin] - Honestly, she'd be first for just her appearance alone. I guess it’s a mix of her clothes, her proportions, and her expression. Also, her sprite is super cute. She's just so animated!:
I also think she'd be way more popular if it wasn't for the fact that she got released in the same event as the Shrine Maidens who kinda overshadowed her.
2. False Hydrangea [Base Skin] - Again, it's her appearance. Not a particular fan of twin tails, but she just looks so cute (though I’ve heard that she looks kinda pregnant because of her poofy dress, but I don’t think it’s that prominent). Also, cute sprite:
Her first and second H Scenes are also some my of favorite in the entire game (especially the second one).
3. Treasure Flower/Gazania [Both Skins] - She’s the only Moneti girl that’s an event unit. Legit all of Moneti’s girls are my absolute favorite in the game in terms of appearance, design, and H Scenes. She’s also the very first event girl that I got because I started my first account during her event.
4. Anemone Flaccida/Flaccid Anemone [Both Skins] - I guess it her design. Her clothes, her proportions, and her pose just looks really attractive.
Least Favorites:
There's actually quite a lot that I don't care for, so I'm not going to bother listing all of them. Honestly, it mostly just the lolis.
EDIT: Woops, forget to put DMM names.