Up until about 740, just do every activity that gives you a Powerful reward that you have time for. Once you hit 740, you hit a point where most powerful rewards will only be +1 light, so they don't do a whole lot for you. At that point, focus on the new stuff for the current season (weekly quests for Benedict, Menagerie, new raid). Those activities will give you rewards that are about +5 light or more. If you finish all that stuff, work on activities from the Black Armory season. They will be +3-4 light upgrades.
The main point here is that not all powerful rewards are equal. In general, the harder and newer the activity, the higher the level of reward.
Edit: Forgot to mention, your drops are based on your maximum equippable light level from your inventory, not just what you are wearing. Keep your highest level stuff in your inventory even if it sucks.
Thanks for the Edit, that is something I did not know!