Gen 5 I guess since every type that existed at the time got at least 3 lines.
Gen V was very fair from this perspective
My ranking:
Rock:
Hoenn I guess.
We have a vast variety like Aggron, Lunatone/Solrock, Armaldo, Cradily, Rwlicanth.
Water:
Also Hoenn. Some nice combinations and heavy hitters like Swampert, Ludicolo, Milotic, Sharpedo, Wailord and much more.
Electric:
Alola I think with nice combinations like A-Golem, A-Raichu, Togedomaru, Viakavolt, Tapu Koko, Zeraora
Grass:
Maybe Kanto. We have Grass/Poison classics like Venusaur, we have a strong Exeggutor.
Poison:
Also Kanto. We have the mentioned Venusaur but also Nidos, Venomoth, Gengar, Tentacruel, Golbat, Muk. There never were so many Poison types in one gen.
Psychic:
Kanto for the 3rd time.
Alakazam, Slowbro, Exeggutor, Mr. Mime, Jynx. Not to mention how OP the type was in the 1st gen
Fire:
Kalos but mostly due to Megas. From standard pokemon we have some good ones like Talonflame and Volcanion. And from Megas we have 2 M-Charizards, Mega Blaziken, Mega Houndoom, M-Camerupt.
Ground:
I think Sinnoh. We have Garchomp, Rhyperior, Gliscor, Hippowdon and Gastrodon.
Flying:
Kalos. Talonflame, Hawlucha, Noivern and some solid Megas: Pidgeot, Pinsir, Aerodactyl, Salamance.
Bug:
Unova. Volcanrona, Scolipede, Crustle, Galvantula, Genesect and some more.
Normal:
I think Kanto. Snorlax, Tauros, Kangaskhan, Chandry, Clefable.
Ghost:
Alola.D Mimikyu, A-Marowak, Pallosand, Marshadow.
Fighting:
Kalos with Hawlucha, Pangoro and Megas: Lucario, Blaziken, Gallade, Heracross, Luponny.
Steel:
Once again Kalos: Aegislash, Klefki and some Megas: Lucario, Mawile, Aggron, Metagross, Scizor, Steelix
Ice:
Sinnoh with some nice type combinations: Mamoswine, Weavile, Abomasnow, Frosslass, Frost Rotom.
Dragon:
Kalos. Goodra, Tyrantrum, Noivern, Dragalga are all solid. Add Mega Altaria, Salamance, Lati@s and top with Rayquaza.
Dark:
I believe that Unova. Hydreigon, Zoroark, Bisharp, Scrafty, Mandibuzz and Krookodile.
Fairy:
Kalos. Not counting re-types we still have some solid Players like Florges, Sylveon, Klefki,Xerneas, Diance. And some good Mega: Garderovoir, Altaria, Mawile and Diance once again.
Total per Region:
Kanto: 4
Johto (my favorite Region:-(): 0
Hoenn: 2
Sinnoh: 2
Unova: 2
Kalos: 6
Alola: 2
It looks like Kalos (Gen VI) had it easy with Mega Evolutions.