R/S/E: Magma Hideout/Route 124/126 Underwater. Clampearl have a 60% Encounter rate. If playing Emerald, every Spawn in the Magma Hideout gives defense evs.
FR/LG: Route 21. 100% Tangela Spawns in grass.
D/P/Pt.: Ruin Maniacs Cave/Tunnel. The Only spawns here are Geodude, or Hippopotas if you find all the Unown.
Hg/Ss: Dark Cave/Route 21. Route 32 side has 60% Geodude. Route 46 side has 50% geodude/Graveler spawns, if post game Route 21 still has 100% Tangela.
B/W: Wellspring Cave/Pinwheel Forest. Cave has 50% Roggenrola, while Forest has Sewaddle and Venipede totaling to 50%.
B2/W2: Relic Castle. Sandshrew and Yamask have a total 60% encounter rate.
X/Y: Terminus Cave. 95% Horde encounters of Durant and Geodude.
OR/AS: Route 111. 100% Sandshrew Horde Encounters.
S/M/US/UM: Exeggutor Island. SoS chain Exeggcute for double the evs per defeated mon.
Sw/Sh: Route 3. The grass directly beneath the Galar Mine has a 99% Rolycoly spawn.
S/V: South Province Area 1/Poco Path. Tarlountula and Scatterbug are common around the fields and trees. Can use sandwich to increase Spawn chances.
Hope this helps.