Legendaries included, Kyurem-B has the highest attack and Kyurem-W has the highest special attack, with both at base 170.
Among nonlengendaries, Mamoswine has the highest attack, at base 130, and Glaceon has the highest special attack, at base 130.
Honorable mentions go to regular Kyurem, who has base 130 in both Atk and Sp Atk, Arceus Ice, who sits at 120 in both Atk and Sp Atk, and Weavile, who in addition to having base 120 Atk is the fastest ice type, with base 125 speed.
Hope this helps :)
