I recommend using Cloak Shot. Deploy cloak and line up with sniper rifle. The last perk for Tactical Cloak improves sniper damage by 40%. Also useful is Incinerate, for armored enemies, useful when you have low ammo and just blasting Husks when they get too close. This is very important. Get your chosen sniper rifle to lvl5 VERY quickly; I can't stress this enough. You'll be one-shotting enemies like nothing.
If you're going to buy alternate powers, I recommend Warp Ammo or Energy Drain. E. Drain comes in handy when you're low on health. You can drain enemies shields/health and gain it back. Warp Ammo does really great damage, even better than Disruptor Ammo (I don't know much about it, so try another guide).
Now onto the important part: choosing your sniper rifle. Everyone has preference: Viper for fire rate, Widow/Mantis for power, etc. Personally, I recommend the Black Widow. It has the power of a Widow (okay not entirely; it's weaker than the widow but it's still able to one-shot almost as good) and it has 3 shots instead of 1 before you reload. Also, the M-98 Widow and the Javelin. If you've played the Widow, you've played the Javelin; they're that similar in stats. If you're the type of player that wants more fire-rate than power, the Viperor Inscisor would be good for you. They do decent damage and have a good firing rate.
Tactical Cloak is your best friend as it's used to escape fire. Cloak Shot (DAT damage boost) sneak-up for melee damage (I guarantee you'll use this power the most). The drawback is that, unlike in ME2, the AI is smarter and they still can shoot you if you cloak up close to them.