R/S/E: Petalsburg City. Marill is the only surf encounter and gives 2 HP Evs
FR/LG: Viridian Forest/Seafoam Islands. If LG, Seafoam Islands has Slowpoke and Slowbro with over 50% encounter rate. Otherwise, Caterpie has a 40% encounter rate.
D/P/PT: Route 201/212. 201 has 50% encounter Bidoof. 212 has the plethora of Wooper, Quagsire, Shellos, and Bibarel on the rainy side.
HG/SS: Slowpoke Well. There's only 1 surfing encounter in the Slowpoke well amd you'll never guess who it is.
B/W/B2/W2: Iccirus City. Only 1 surfing encounter, being Stunfisk. Fishing in ripples also nets Seismitoad occasionally.
X/Y: Connecting Cave/Route 6/ Route 20. Each location has a Horde encounter with a 60% chance to summon Whismur, Gulpin, and Foongus respectively. Could also just use Super training.
ORAS: Rustruf Tunnel. 100% Whismur Horde battles. Could also just use supertraing.
S/M/US/UM: Route 1. SOS chaining gives double the amount of evs. Caterpie has a high call chance to summon an ally and are very easy to KO.
Sw/Sh: Mototoske Outskirts/Route 1. Noctowl has a high Spawn rate for Outskirts, Swovet is the same for Route 1.
S/V: South Province Area 1/Poco Path. A wide variety of Hp giving Pokemon I'm this area. LeChonk around the fields, Azurill along the riverbed, Igglybuff on the city outskirts and Paldean Wooper by the lake near Los Platos.
Hope this helps.
