Man why are there so many Zed/Sivir decks today? I mean I am cool with that but please wait until next week before spamming Sivir decks!
I feel like at this point pretty much every region combination has a viable/decent deck. I think only two exceptions I can think of are Demacia + Noxus (its decent but kinda vanilla), Demacia P&Z and Bilgewater + Ionia (used to have Elusives with Fizz but weaker now).
Targon
Taliyah/Malphite Landmark deck for Shurima
Shyvanna/Asol Dragon and Taric/Jarvan deck for Demacia
Overwhelm or Draven or Riven + Taric with Noxus (this one is generally a pile but has been competitively viable)
Lee Sin/Karma with Ionia
TF/Aphelios or Soraka/Tahm with Bilgewater
Targon's Peak Asol with Freljord
Asol/Thresh and Nightfall with Shadow Isles
Rubin Pile with P&Z
Shurima
Lurkers with Bilgewater
Azirelia + Zed/Sivir in Ionia
Aggro/Burn with Noxus
Thresh/Nasus with Shadow Isles
Ekko/ZIlean with P&Z (ok this one needs some buffs and more finishers but there is something here)
Sivir/Gaven or Jarvan or Renekton with Demacia
Overwhelm or Turbo Thralls with Freljord
Demacia
Shen Jarvan with Ionia
Lucian + Draven aggro or Riven + Draven Field Promotion memes with Noxus
Mono Fiora, Poros or Yetis with Freljord (generally a pile of stats deck)
Matron Cithria or Lucian/Hecarim Plaza with Shadow Isles (and maybe SI Dragons in the next expansion)
Scouts with Bilgewater
Heimer + Lux or Demacia P&Z - Ok this one is pretty bad. Feels like this one should have worked with Lux but it really doesn't. I saw someone mess with Ekko Vi Demacia though.
Freljord
FTR + Anivia Control with Shadow Isles
Ashe, Leblanc Yetis, Swain Sejuani/LIssandra, Vlad/Braum with Noxus
Teemo Sej/Ezrael Hex, Yetis and Poros with P&Z
Zed Elusives or Feel the Mina with Ionia
Plunder/Nab with Bilgewater
Noxus
Aggro Burn or Ez/Draven with P&Z
TF Swain or Pirates with Bilgewater
Riven/Draven OTK or Yasuo/Swain (copium) with Ionia
Spider Aggro with Shadow Isles
P&Z
Corina Control, Prankster/Chirean Burn with Shadow Isles
TF/Ez or TF/Fizz with Bilgewater
Ez/Karma, Lulu Jinx or Heimer/Vi with Ionia
Shadow Isles
Deep or Go Hard with Bilgewater
Ephemerals or Spooky Karma with Ionia
Bilgewater
Elusives, MF/Irelia or Fizz/Lee with Ionia (mediocre pile mostly)
(Obviously I didn't list repeat combinations here and obviously there are more than 0 viable Ionia decks lol)
It seems that Shurima and Targon are the two regions that have worked the best with the largest number of regions. Probably because they have harder pushed regional archetypes like Lurks, Sand Blades, Dragons, Landmarks, Predict and Tahm/Soraka.