Just curious to know.
IMO it's kinda hard to chose because they all are really cool if you think about it, but for me it would be Elderwood. Especially because of Hecarim and Bard's splash. When I see them I can totally imagine a story behind these splash.
http://2.bp.blogspot.com/-yIQRxtX7M5w/VPgAHYJ7FxI/AAAAAAAAlLY/WY_ZYLiBP1E/s1600/Bard_Splash_1.jpg
You like the Elderwood skins, I see.
http://31.media.tumblr.com/3a9146444df7ce61f0667429d8594d03/tumblr_ner0yxdSDb1qeclafo1_500.gif
My favorite skin line would probably be the Battle Boss line, even though it only has Blitzcrank and Veigar.
My top 3 skin lines are:
Elderwood: I love how it turns them into mythical guardians of nature and even tho I don't really play any of the champs who have an Elderwood skin, I would love to own them (especially Hecarims)! I hope Riot keeps making skins for the line, I think Kindred and Varus are some good candidates.
Blood Moon: It's so cool seeing those champs turned into demonic servents of the Blood Moon! Their red color scheme combined with their white masks is just awesome. I just recently made a thread about some potential new Blood Moon skins, like BM Diana, Jhin, Syndra or Kindred, but I can think of way more potential candidates like Yi, Vlad or Bard.
Guardian of the Sands/Risen: Whitout question my favourite skin line! Shurima is my favourite region of Runterra for many reasons and so I love seeing champs turned into mythical shuriman creatures like Kha and Fiddle, but I also like the alternate stories it provides for shuriman champs, like Xerath turning into a guardian of Shurima. Sadly the most skins are for junglers and I main support, but I will definitly get GotS Rammus, when he goes on sale, since Rammus was the first champ I ever bought and is still one of my favourite today. I hope Riot will tells us eventually if the GotS and Risen skins are the same (since Guardian of the Sands Fiddkesticks is pretty long, so Risen was just the better option) or if they are different skin themes...for example the Risen skins could be the enemies of the GotS, so they guard their tombs in order to never let them rise and thereby threat the sands again or they could even be there masters. And I really hope we will see new skins for this line in the future, GotS Rek'Sai would be perfect and GotS Galio is something I could see working very well! As for a Risen...I found this awesome skin concept for Lissandra: https://67.media.tumblr.com/b78d43eca437858ecc090dd41f155b88/tumblr_o9kho4mT8S1vuu3clo1_540.png
I would say dragon trainer, combining the bot lane combo of Tristana and Lulu and hoping that Riot would do more skins for yordles for Teemo, Rumble and Heimer for the same skin line...
Doubt it will happen since Teemo got too many skins, the last one being completely disgusting and undeserved to good players...
PROJECT.
http://67.media.tumblr.com/bf056982d7478d5c9d62480a8d7806e2/tumblr_nuds98uP9R1r10reao2_500.gif
Battlecast is a close second.
The RPG one and Worldbeaker both seem kind of awesome.
I like the battlecast skins personally.
Also, even though you didn't ask for it. Academy skin line is DEFINENTLY the worst skin line rn.
If Ravenborn were a skin line, that would be it.
no love for Arcade
How dare you leave Elderwood LeBlanc!
But for your question, I'd go with the Ravenborn set if they add another one :)
The arclight and headhunter lines have to be my favorite skin lines! I also think the warden vs marauder line is pretty awesome, it does a great job for a 750 RP line.
Dark Star, Gothic Horror, Mecha, and all the joke skin lines. It's hard to pick just one.
http://pm1.narvii.com/5829/7857ad3c331604aa1b808ce6a70f5aa4c8aa0c49_hq.jpg
Dragon Slayer, Still need one more champ. Although it isn't a skin line it need to be Cosmic Reaver Kassadin is the best.
Project.
For me it is Elderwood, if we just say thematic and splash art. I don't like elder wood Leblanc that much though.
But if we say I game and story I would say project. I really like how riot gave the latest project skins roles in a story. Ashe as the leader that fights for freedom and Ekko who's job is to "get them in" in their missions. Don't remember katas role though.
Mine is the western themed ones (High noon and longhorn alistar)
i love the elderwood skins too for the same reason and also because of the colors. Although I dont really like the fact that when you're playing let's say elderwood hecarim, your model is green-brown but the ability icons and the "portrait" are grey/blue :(. My other 2 favourite skin lines would be dragon trainer and eternum (rito pls eternum/elderwood 
)
Wish Cosmic Reaver was a skin line, cause that skin is da best!
Pool party is where it's at. One day, every champ will have this skin, except urgot. Crabgot is as close as hes allowed.
