For me, I'd have to say Warwick's. It just suits him so perfectly and hearing the music, you can feel his uncontrollable rage. It sort of feels like it's coming at you in some strange way. I've even saved it on my phone to listen to it whenever I like.
Other ones I like are Taric's, Yorick's, Taliyah and Lucian
What's your favorite login screen music to date?
Definitely 
Elementalist Lux is really nice, but I think the obvious choice is Jhin.
There are so many great ones...
Daylight's end (Diana Login Screen) is probably my all-time favorite. It is perfection. From the recent ones, I probably enjoyed Camille, Taliyah and Warwick the most. Also the Bilgewater Soundtrack was also amazing, I love it so much. Elementalist Lux and the recent Dark Star are also too good for this world. ...I really have download more songs from them and put them also on my MP3 Player.
Honest, the sound team are basically outdoing themself every time. No matter who much praise they get, it will be never enough for their work.
It may seem like an odd choice, but I am quite fond of Ryze's login theme. It is haunting and mysterious, and I think it captures the image of a world-wearing wandering wizard perfectly.
PROJECT Ashe is also impressive. It combines the techno-style of the PROJECT universe with the lofty heroism of Ashe's character.
Taliyah, Jhin, Camille, and Aurelion Sol are also great.
Kindred > all Arcade > the rest
Ekko's login theme was the best IMO.
Tossup between Diana's login theme and Dragon Trainer Trist's.
Definitely Jinx and Camille's. Warwick's is pretty high up there too.
I love Kled's and Ivern's themes.
Aurelion's and Tahm's is the best. They suits them PERFECTLY
My all-time favourites are Diana's, Taliyah's, Camille's, Jhin's and Warwick's.
I like the honor one. I am still waiting for Riot to put the whole theam on their youtube channel.
Probably a toss-up between Camille's and Kindred's.
Diana simply for the lyrical music. I'm hoping we'll get a Noxian login one day with some lyrical war music, all 19th century esque :P Special mentions would be the original theme login and the Dominion login remix of it.
Season 6 login, that opening with Caitlyn firing a bullet was so badass
