It's a broad and not entirely clear question, but we'll endeavour to answer.
I have two preferred classes to play in Mass Effect 3, one for the single player campaign which is a continuation of a ME2 save, the Infiltrator, and my preferred class on the multiplayer campaign, the Engineer. In both instances I try and keep the specific shield / biotic destruction abilitys and the armour destruction abilities maxed with full damage, adding AoE effects if it doesn't detract from damage too much. I usually don't both with the damage vs frozen targets when it comes to Incinerate, preferring instead the damage vs armoured foes as it possesses a more widespread use).
Most abilities with the Infiltrator class I gear towards sniper rifle usage, increasing head shot bonuses and slow down abilities to ensure the head shots can be set up. Also, with it being single player, I tend to increase health and shields. Weaponwise I use a high powered sniper rifle, machine pistol and assault rifle allowing for long and short range combat.
With the Engineer I find the Combat Drone to be highly useful, not just as a means of increasing damage but also as a 'seeker'. By placing it in the centre of an expanse it can be used to seek out enemies as it begins travelling towards them as soon as they are detected. With that in mind I keep this maxed, using rockets and AoE electrical blasts. Unlike my single player save, shields and health take a 4th place priority. I arm my engineer with rapid fire and short range weapons, such as the Geth energy shotgun and machine pistols.