R/S/E: Abandoned Ship/Route 113. Surfing or Fishing with the Super Rod Guarantees Tentacool or Tentacruel. If Emerald or Sapphire Route 113 Has Lotad and Lombre in addition to Swablu, for 90% total encounter rate.
Fr/Lg: Cinnabar Island/Route 20/Route 21/12. All of these locations have 100% Tentacool spawns while surfing.
D/P/Pt: Valley Windworks/Route 223. Both have Tentacool/Tentacruel at over 60% encounter rate while surfing.
Hg/Ss: Cherrygrove City/New Bark Town/Cianwood City. All locations have 100% Tentacool/Tentacruel encounters while surfing.
B/W: Driftveil/Route 4/17/18. All areas have 100% Frillish encounters while surfing.
B2/W2: Virbank City. Also has 100% Frillish encounters while surfing.
X/Y: Route 7/Reflection Cave. Both areas have 60% Horde encounters for Hoppip and Mime Jr respectively.
OR/AS: Route 115. 100% Swablu Horde encounters.
S/M/US/UM: Route 7. SoS chain Tentacool for double the evs per mom defeated.
Sw/Sh Circestre Bay. Fishing or surfing has a high chance for Mantykes and Mantines.
S/V: South Province Area 3. Spoink is a fairly common encounter here, it is also the only psychic type on the Route so using a sandwich will increase its Spawn odds drastically.
Hope this helps.