Whichever one annoys you the most going up against. You get a decent Mech and can learn how its special ability works and its weakness to it. 

Just remember if you are spending cash and increase your hanger power, you will still end up going up against mechs and such that are more powerful. There is no magic mech/weapon unless your whole hanger is maxed out. Spend wisely with knowledge. 

Surge.

Surge all day every day, even after the nerf.