Porygon-Z can be level one if you trade holding the upgrade and then again with the dubious disk. It has a base 135 S.Attack, the highest any Non-legendary that can be a lvl. 1 (through an event in HG/SS you can get an egg with either Giratina, Dialga, or Palkia, and Dialga and Palkia have 150 S.Attack). I am not sure the EXACT stat at level 1 but it would be the highest without special boosts(EG. Clamperl like GhostTypes said). He also has a great movepool.
Escavalier has the highest non-boosted Attack of all Pokemon capable of being level one. 135 base Attack (the same Base Attack as Porygon-Z's Base Special Attack). Also if this is a lvl 1 battle he can learn Mega-horn by breeding which could prove VERY useful.
Light-ball Pikachu has not only high Special-Attack but High Attack as well, so take him into consideration.
Also if you plan on using Escavalier and this is doubles, then I reccomend using....
Vaporeon to cover his weakness to fire, Vaporeon learns some of its best moves by TM and Breeding which helps with lvl. 1 battles.