Choosing which armor parts to use (at low lvls)

Abbreviations:
ar dur = armor durability
es dur = energy shield durability
ar = armor resistance
esr = energy shield resistance
hp = hitpoints
dur = durability
res = resistance

Something unique in this game is that your "build" does not depend on your character's stats, but on your mecha's stats. So basically, any race can use any build, and swap between builds just by swapping armor parts.

Till level ~30, the best setup to use is arms and legs with hp bonuses, and the rest of the body parts with ar dur or es dur (ar dur recommended for enhanceds, es dur recommended for neos, natural humans can use any at this stage).

Past lvl ~30 though, the build should be changed to a res build, since dur starts to fall behind (dur works better at lower levels, since its bonuses overweight those of low lvl res parts). Usually the hp arms and legs are kept, and the parts which had previously dur bonuses, are replaced with res parts. If needed, the legs can also be replaced with res legs. Here is a list with some of the best parts for a basic damage/tanking type build (I don't know the Mars locations for these):

-- Head:
------ rank 2 +4 ar/esr (dropped by Anillas Guard Officers & Logu Path Assault Commanders)
------ rank 1 +2 ar/esr (dropped by Domier Skywalkers and Headman of Darlon in Osider, hard to find, randomly dropped also by low lvl mobs in Domier & Osider)
-- Arms:
------ rank 2 +35% hp to shooting/combat weapons (dropped by Domier Annihilators & Osider Assaults)
------ rank 8 +240 (without %) hp to shooting/combat weapons (Sanbuick Lair Vultures, South Saints of Darlon, temporal use only)
------ rank 17 purple arms from Luke (for the hp & damage bonus)
------ rank 3+4 ar/esr arms (dropped by the lvl 19 Cargemiller Floater & Soer Cleaner)
------ rank 1+2 ar/esr arms (dropped by random lvl 1-5 mobs in Domier & Osider, hard to find).
-- Body:
------ rank 1 +2 ar/esr, its dropped by Domier Guard Commanders & Pontiffs of Darlon in Osider.
-- Engine:
------ rank 7 +4 ar/esr, dropped by Alcaleph (last boss in Exotic Planet, easy to get)
------ rank 8/9 +8 ar/esr, dropped by Dracula (last boss in FB20 Space Base, way harder to get but better than the 7+4 one).
-- Legs:
------ rank 7+50% or rank 8+55% hp to shooting/combat weapons (dropped by Flower Bud boss, 3rd boss in FB15 Exotic Planet)
------ rank 4+6 ar/esr (dropped by Cargemiller Fury - boss version, & Soer Gunnery Commander)
------ rank 1+2 ar/esr (dropped by random lvl 1-5 mobs in Domier & Osider, hard to find)
-- Core:
------ rank 5+20 esr (quest reward, only useful if going esr build, which isnt that useful at endgame/close range pvp/pve)
------ rank 7+10 ar (quest reward)
------ rank 10 +10 ar/esr (dropped by the Sanbuick Lair Monster King & South Ironophila Maggot King - credits to airstrife for the Mars version)
------ rank 15 purple core, +10 ar and +50% armor dur, dropped by a boss in FB45.
------ rank 16 purple core, +12 ar and +60% armor dur, dropped by a boss in FB40.
------ rank 17 purple core, +15 ar and +75% armor dur, dropped by last boss in FB35 and one in FB45.
------ rank 19 set purple core, they have ~80% hp and ~15 ar/esr, obtainable from tb2 I believe.
-- Cabin:
------ rank 9+10 ar (quest item for enhanceds only, for temporal use)
------ anything with large amounts of dur, such as rank 10+600 dur dropped by Andrew Arbiters (temporary cabins anyways)
------ purple rank 17/18 cabins dropped by the north west boss in Luke (they have large amounts of durability)
------ purple rank 17 cabin, +10 armor res & +50% ar dur, dropped by FB40's King Kong.
------ purple rank 19 set cabins (with +hp% and +resistance)
-- Jetpack: (can be combined with wings to have more stats):
------ Cash Shop perma wings if possible
------ any jetpack with large amounts of dur (1k dur max), from Dark Valley FB40 (& 45 I guess), though the dual lock jetpacks may be a better option in the long run, obtainable in the same Instance.

While there are many other builds (interval, critical, sniper, etc), resistance is a must for high lvl players, so it will be helpful regardless of what class you are. The other builds are explained in detail in the Builds section of the guide.