Yanmega.
Do not use Ninjask for offence. It is easily outclassed, and one of the Pokemon that outclasses it is Yanmega. Ninjask is better off sticking with Baton Passing Speed and Attack to it's team mates.
Yanmega might not have the most impressive Speed ever, but after a single Protect it receives a 1.5x boost to it's Speed with Speed Boost, enough to outpace everything that is a threat to it. If you are looking for an attacker, Yanmega is better than Ninjask.
Yanmega @ Life Orb
Trait: Speed Boost
EVs: 252 SAtk, 252 Spd, 4 HP
Timid Nature (+Spd, -Atk)
- Protect
- Bug Buzz
- Air Slash
- Hidden Power Rock
Choice Specs is also a good item for Yanmega, although the standard Protect set is generally better.