R/S/E: Route 102/117/Petalburg City. All locations have a 100% chance of fishing a Corphish when using super rod. If playing Sapphire or Emerald, Mt. Pyre floors 1-3 have guaranteed Shuppet spawns.
FR/LG: Mt Moon B1F. 100% Paras spawns.
D/P/Pt: Twinleaf Town. Super Rod fishing in lake nets only Seaking and Gyarados, 2evs each.
HG/SS: Route 42. Surfing only yields Goldeen and Seaking.
B/W: Dragonspiral Tower Interior. All encounters give Atk evs.
B2/W2: Dragonspiral Tower Interior/ Route 4. Tower is the same as above, just higher level. Route 4 has Sandile, Scraggy, and Darumaka making up 75% of the encounters.
X/Y: Route 20/14. Both have 60% Weepinbell/Bellsprout Horde encounters while also having 5% Arbok/Evans encounters.
OR/AS: Mt. Pyre. 100% Shuppet Horde encounters.
S/M/US/UM: Route 1. SoS chain Pikipek or Yungoos for double evs per defeated mon.
Sw/Sh: Route 8. Pawniard, Gurdurr, and Golett are over half the spawns.
S/V South Province Area 3. Shinx and Yungoos spawn in abundance between the Pokémon Center and Mesagoza. Further down Growlithe spawns occasionally as well. Can use a sandwich to Increase encounter odds.
Hope this helps.