For our Narrative Campaign's force, Jumping Jack is the most common one, especially as most of our force has Jumping Movement, even having modified an Awesome to Jump.
Human TRO is our next most common SPA (one of our players REALLY likes it).
Sniper is our next most common SPA, with one person having respecc'd in to it from Range Master: Long.
We also have a Valkyrie pilot who has Oblique Attacker, and Sandblaster: LRMs. Seriously, if I had planned my progression better, I may have gone a similar route in mech and SPA selection.
Still, Jumping Jack is probably the best thing for a very mobile force. It makes one of my pilots an absolute terror in her Mercury II. We also use Hit & Run for one of our Force Command Abilities, and Jumping Jack helps when we are not outnumbered.
Melee Specialist and Melee Master can be truly brutal.
Adding Fist Fire in to that mixture is just NASTY, too.