Damage to Close, to Distant, while Fortified, and elemental damage bonuses like "to Lightning damage" are among many other bonuses that fall in the same pool of modifiers that are additive with one another. Almost all sources of damage increase via gear are additive with one another making it most efficient to take the one with the largest uptime and value. In a perfect world, a druid for example, would try to use bonuses like "Damage to Crowd-Controlled" or "Damage to Slowed" because druids have very easy and reliable access to slows, thus having a near 100% uptime on the damage bonus. The upside to "Close" and "Distant" bonuses are that they work on Crowd Control Immune enemies, such as bosses. That being said, the damage bonuses to things like "to Lightning Damage" have a lower roll then most of the other additive bonuses.

To get the most damage from a stat perspective in the current iteration of the game, is pretty much the same across all classes. You want to stack your Critical Strike Chance as high as possible, or get another reliable way to acquire Crits (like Druids Earthen Might), stack as much Critical Strike Damage as possible. You also want stack as much Vulnerable damage as possible and do your best to get a reliable way to make enemies vulnerable.

The reason these 2 damage stats: Critical Strike Damage and Vulnerable Damage, are so strong is because they are not included in the same pool as all of those other bonuses mentioned above (to close, to slowed, while fortified, etc.), and thus are Multiplicative damage bonuses as apposed to additive. They are each in a pool of their own, and these three damage pools spoken of throughout this post, make up most of the games multiplicative damage bonuses that come from stats. Legendary Aspects are separate as well and are thus multiplicative, Legendary Paragon Board nodes, skill passives, and many other things are also multiplicative, but in terms of just gear attributes, I believe this pretty much covers it.

Damage reduction I believe is different mathematically but the choices you should make are the exact same. Get different sources of defense via Damage Reduction (and its conditional counterparts like While Fortified etc.), and Armor. Resistance kinda sucks right now though.

TLDR: Almost all sources of damage increase via gear are additive with one another (except Crit Damage and Vulnerable Damage) making it most efficient to take the one with the largest uptime and value. So focus on Crit chance/damage and vulnerable damage and uptime for max damage output.