Well. I would honestly recommend avoiding treating sniping as a "career" as it's a pretty unhealthy role in dust, often placing people far back from objectives, not hacking anything and taking potshots at the rare enemy that wanders into sight of their 'safe' zone...

But if you want the 'best' sniper you can get in terms of damage, it would be the caldari commando, loaded up with as many complex light damage mods as you can get, your sniper rifle of choice, a rail rifle of some variety and a reactive plate or two. Be forewarned that this is incredibly hard to fit as caldari commando's are kind of short on CPU right now.

There's a lot of romantacism and mystique built up around snipers in shooter games, but to be brutally honest... sniping in dust is in a *very* bad state. It requires you to be an absolutely AMAZING shot to score anywhere near well... and if you're that good of a shot you could pick up prettymuch any other rifle in the game and slay even harder while playing a much more involved role for your teammates both in terms of support and objectives. A bad sniper is often resented by teammates, a 'good' one is still often unnoticed because the role doesn't really play objectives.

To explain a little bit more about the Caldari commando, rather than just rant about how much I personally dislike snipers:

The Cal.Com gets a 25% reload speed bonus at max level to all light weapons (this is quite good for some) and then it gets an unpenalized 10% damage bonus to light rail weaponry (rail rifle and sniper rifle). Because this bonus is unpenalized ot both applies ahead of damage mods and, it provides a greater bonus than 'usual' with damage mods and it stacks with the warbarge damage bonus too.

Where 3 complex light damage mods on any other suit would bring you up to around 18% increased damage, on the caldari commando it brings you up to around 30% increased damage (with rail weapons only). The commando does have some large drawbacks to it though, with its low shield recharge, large hitbox & scan profile and non-existent electronic warfare capabilities.

Here's the lowdown on stacking penalties with a handy calculator. http://wiki.dust514.info/index.php?title=Stacking_Penalties in general it goes unpenalized to penalized, highest # to lowest number (1st mod = 100% listed value, 2nd mod 87%, third ~57%, fourth ~28.5%, fifth 12.5%, these are multiplied with each other so in some cases even the fourth and fifth mod can be quite good, where in other cases like the krins damage mod the fourth and fifth one don't even add a full percent to your damage output).

On more open maps I really enjoy running a tac sniper & standard rail rifle, as with a few bodyshots you can really put the hurt into someone and you don't need to worry as much about getting exclusively headshots to do damage.