Kanto:
Brock can be found in Diglett Tunnel wanting to trade his Rhyhorn for Bonsly.
Misty can be found on Cerulean Cape.
Lt. Surge can be found wanting to trade his Foreign Pikachu at the Power Plant.
Janine can be found in the Celadon Department Store arguing with Falkner, alongside being photographed in the Badge Check Gates. She can also be found in FRLG.
Sabrina is prominent in the Pokéstar Studios.
Johto:
Falkner can be found arguing with Janine.
Whitney introduces the Pokéathlon and is a competitor.
Morty is in the Burned Tower.
Janine is in the Glitter Lighthouse and competes in the Pokéathlon.
Clair has the whole Dragon Den thing.
And all of the above are disregarding the Phone Call Services, Rematches, and PWT.
Hoenn:
Wattson appears in Mauville to request help for the generator in New Mauville.
Sinnoh:
Roark appears in the Underground on the first visit.
Gardenia appears in Eterna Forest, in front of the Old Chateau.
Fantina is a contestant in Pokémon Contests, and has been stated to often visit the Lost Tower.
Maylene appears on Route 217, Snowpoint City, and the Villa, and appears in HGSS.
Crasher Wake is outside the Gym quite often in DPPt, and is also in HGSS to give Masks in the Celadon Department Store, and B2W2 on Route 18.
Byron appears on Iron Island to give an Iron Coat and some back-info.
Candice appears in the Villa and grants access to the Snowpoint Temple.
Volkner appears in the Sunyshore Lighthouse and in the Villa.
Unova:
All BW Gym Leaders except the Straiton Gym Leaders (who have a Memory Link in B2W2) appear in N's Castle.
Roxie appears multiple times in Virbank.
Marlon helps out with stuff, and so does Cheren.
Kalos:
Viola introduces the Battle Chateau.
Korrina appears in the Tower of Mastery.
Clemont appears outside the Gym, albeit briefly.
Wulfric appears in Pokémon Village.