For starters you can't have everything. If you train in certain stats, then others have to lose out a little.
I'd recommend choosing either attack or special attack to focus on, that way you can get it really high.
For special attack, beat Gastly/Haunter in the Old Chateau.
For attack, try Route 210, you'll find Bibarel, Machop, Machoke and Scyther.
For Speed, there are Starly on Route 201 and some speed-giving pokemon in the Trophy Garden.