Is the generic Sniper Rifle up for consideration? It's about the best combination of hitting power, range, and availability in the ballistic category. Another reason to go ballistic are the options for special ammunition. An AP round in a sniper rifle WILL go through pretty much any personal armor that isn't power armor. Exploding or flechette rounds are great for soft targets, and if you value stealth over hitting power, subsonic rounds make it harder to hear the shot. You could even get tricky and use airburst or incendiary ammo, if you wanted.
I wouldn't suggest gauss guns. They're expensive and extremely rare.
Laser weapons... they have great range though for most of them the beam can end up pointing right back at you. The Starfire ER Laser Rifle is purpose built for assassination. It has the best range of any inner sphere laser rifle, and its beam is harder to spot. Downside is that it has a five second (one turn) recharge cycle between shots and it's painfully rare outside the free worlds league.