Definitely Midnight! It's one of my favourite Pokémon. I think it looks super cool and has a ton of personality. Midday is too bland for me. I mean, it's alright I guess, but It just isn't as interesting or awesome as it's counterpart. And Dusk, well, it just sucks. Design-wise at least. I've never actually used one. It's literally just Midday with a different colour palette and a bit more fur around its neck. You can't get more lazy than that...
I personally like the midday and dusk form over the midnight form, A) I don't like werewolves, b) I like regular wolves, C) I liked the design of the midday and midnight forms.
I think Lycanroc in general was given the same treatment as Greninja, but for no actual reason, which surprised me. At least Greninja is a starter, but Lycanroc is just... there. That being said, I prefer the Midnight form just because of how much more sinister it looks. I love its design, the glowing red eyes and huge paws and mane. It just looks awesome.
Not really a big fan of either tbh, but if I had to choose it would be Midday, I'm a big fan of hitting first and hitting hard.
I actually like both design-wise. Though, if I had to choose one for a fight, it'd be Midday form Lycanroc, since it's a lot faster.
Has to be Midday for me not only as it looks like an actual evolution to Rockruff but also a mainstay on some of my competitive teams.