Here is a list, up to date as of gen 9, excluding megas and legendary Pokemon, showing the top 5 Pokémon in Sp. Attack (excluding legendaries), with an option for Pokémon obtainable in BW/DPP:
1st:
- Overall: Xurkitree at 173
- Obtainable in gen 4/5: Chandelure at 145
2nd:
- Overall: Ash-Greninja at 153
- Obtainable in gen 4/5: Darmanitan (Zen Mode) at 140
3rd:
- Overall: Tied bewtween Cursola, Vikavolt, and Chandelure at 145
- Obtainable in gen 4/5: Volcarona at 135
4th:
- Overall: Tied between Iron Moth, Wishiwashi (School Form), Aegislash (Blade Form), and Darmanitan (Zen Mode) at 140
- Obtainable in gen 4/5: Tied Porygon-Z and Alakazam at 135
5th:
- Overall: Tied between Raging Bolt and Pheromosa at 137
- Obtainable in gen 4/5: Tied between Glaceon, Magnezone, Espeon, and Gengar at 130
Source: Pokédex with stats--Pokémon Database
Hope I helped!