Attack and Defence are always the most difficult for EV.
HP - Whismur in Rustboro Cave give 1 HP EV each
Attack - Ruby only - higher floors of Mount Pyre contain only Shuppet, for 1 Attack EV each
Defence - Desert area for Sandshrew. Run away from anything else.
Speed - Route 118 - contains Zigzagoon (1 EV), Taillow (1 EV), Electrike (1 EV), Manectric (2 EV), Wingull (1 EV)
Special Attack - Route 113 contains Spinda for 1 EV each. Run away from anything else.
Special Defence - Surf on any water route, and fight Tentacool for 1 EV or Tentacruel for 2 EV. Best place for this is in the small water area in the Abandoned Ship, since only Tentacool/Tentacruel appear and nothing else.