Not a lot of bad things to say about the Needle Rifle with a targeter on it and the accuracy rules errata, not a lot lives through up to 4D10 of damage.

Silent, no muzzle flash, no windage required

The first two will stop angry people you're shooting at from dropping artillery rounds on you, the last makes shooting much easier over long ranges. About the only real drawback is the price of ammo and its a pretty distinctive way of making yourself well known if people (that don't appreciate you shooting them) find out you're in the neighbourhood carrying one.

If that's an issue I still standby the old Hunting rifle, some kind of flash suppressor, scope and variety of nasty specialist ammo you can throw through it, all for under 250 thrones with enough ammo to kill a company, they're everywhere and they hurt a lot. Most average, lightly armoured NPC's will probably die from 1 aimed shot, most PC's will probably be dead in 2 aimed shots, in most cases.

Another excelent sniper is the Ranger Long Rifle it has the same profile as a long Las but a basic range of 200 metres plus a penetration of 6 if you use a full action on aiming. and here i am fair saying that you do not use Imperial upgrades on Eldar tech but if you can this weapon is getting really nasty.

Though we do not have the rules for it (Yet, but there is rumours it will be in Ascension), the best Sniper in the entire 40k univers is the Vindicare Exitus Rifle, End of Story.