It depends what do you enjoy the most in DB.
I’m pretty bored with fragger and javeling, but if you like your k/d ratio on top, then pick one of them, jav doesn’t require skill, easiest merc, fun to play but I got bored pretty quickly.
Sparks - again, this is one of the funniest to play merc around, hide behind your teammaes, revive them, run like the Flash and hit everyone with a cricket to death.
Nader - nothing special I guess, but also fun to play.
Kira with katana and her laser is also fun to play, you can either play for kills, gamexp destroying ex or fun with katana.
Guardian - under powered, kinda powerless but useful in 1 out of 10 cases
However, if I were you and short on credits, I would wait for an update to see if Hunter is worth his name. Remember he’s [will be] a recon class (somewhere inbetween Phantom<>Sniper[aimee,vas,red])
note:
This is just my opinion.