Your Elite Equipment gives good boosts in battle power and also increases your damage in the Tower Defense mini game. Whether your focus is to be a Long Range or a Mid Range player, the top 4 in your cities' Elite Equipment should be your initial priority. These are (in order of importance for a Long Range player) - Radar, Computer, Apparatus, and Reactor. OR (in order of importance for a Mid Range player) - Reactor, Computer, Apparatus, and Radar.

Please refer to this YouTube video from user LumberghAOO, which is a great in-depth explanation of all things pertaining to equipment and materials. This YouTube video from Feral Vincent is good, as well. Subtitle translation and, in some cases, dubbed audio can be found within the video's "settings" menu

Tip: Prioritize leveling your Damage Reduction/Damage Increase Equipment Extensions which reside in the top 4. These are very important.