All answers are almost entirely subjective
I'm just bored but I don't know what to start with on the Universe page. Which champions do you think come from the coolest background?
Bonus: Who would make the coolest cinematics/songs based on them (like the theory that Mortal Reminder is about Varus)
My personal route would be to go to the Bilgewater page, and read both Burning Tides, and Shadow and Fortune. They tie into each other pretty well, and many of the champs who feature in both stories have updated stories and bios. There's also a pretty neat clickable 3-part picture of the slaughter docks and Gangplank's ship which gives you a bit more info about the world.
It's good to get up to date on where Bilgewater is, what Miss Fortune is up to, etc. seeing as she's gone through quite a lot in those two stories!
(I also agree with Fasmodey, LordHippoman, and others that Shurima is a good one too.)
Shurima is the most developed. Bilgewater had the BIGGEST lore event. The Shadow Isles is the most tragic and mystery filled. Zaun and Piltover are a place where you can only imagine what comes next. Freljord has the largest conflict currently going on that is the most mortal and relatable (although they have their fair share of insane beings)
It's up to you to be honest.
Zaun because who doesnt love a gritty steam punk city filled with violent vigilantes and a crossbow wielding rat. Also shurima because renekton's lore is possibly the saddest in the game and also people there literally roll around like a bunch of idiots in order to gain the favor of everyone's favorite armordillo.
Shurima.
I love Shurima and Piltover, Orianna and Taliyah especially. Ahri's lore is really nice too.
Shadow isles. I like kalista's.
Noxus. Expanding empire, political machinations, the potential for actual battles and tactics that hopefully make sense. It's got everything I like.
So long as it doesn't turn into a big nothing about which magic user is using their magic to control everything instead of actually going through the interesting steps on how to be a political mastermind. I'm good.
Bilgewater has consistently hit it out of the park with all of their new Lore content. Shurima's pretty solid too.
Shurima for me, the scale of the lands, the diverse champion line up from there and how new ones can be introduced is so much better than the other regions.
I mean Shurima has the Ascended (we need more of them btw, we only have 4 in game). Skarner (one of the best stories so far), Rek'Sai, Amumu, Rammus, standard humans with magical power (Taliyah). What have others got? a bunch of humans? Dull. Plus based on the recent lore, it sounds like Shurima will be the start of things going haywire in Runeterra. A once ancient and dead civilisation returning, it's people uniting and lead by an all powerful being with an army? How long do you think the Noxus controlled cities in Shurima will stay Noxus controlled? Will others stand for a mad ascended running around trying to 'conquer them'? (Xerath).
For me, Aurelion Sol's lore si the best one.[slayer-pantheon-thumbs]
was ionia and shadow isles were only s tier zones but with the new lore with anivia, ornn, and volibear. freljord has entered s tier with them imo
Shadow Isles because 
Shurima!
I won't rest until my boy Varus gets some lore love.
Ionia or shadow isles, it's a tie
