TLDR: Ninja Garb or Necrosheild Suit for F2P, Emergency Shell for P2P.
When you start playing, use whatever armour gives you the most health regardless of the type (e.g. if you got a Rare or Epic armour from a chest, use it) - because of the way Defense is calculated and because you don’t have much in the way of Melee & Ranged Resistance permanent upgrades, the difference between Body / Tactical / Heavy is negligible (the 10%/20%/30% is a boost to your Resistance, not protection against damage - details here).
As you progress through worlds, you could get a small edge by selecting Body / Tactical / Heavy Armour to match the world and/or the enemies you find difficult to kill. However, unless LightHeart adopts this idea to make Melee/Ranged Resistance and Armour to act as a percentage defense (and maybe even if they do), at higher difficulties it is better to increase the chance of avoiding damage than reducing the damage taken, for which there are 3 options:
- The Ninja Garb boosts your chance of dodging by 7%, which in the early stages means more or less doubling your chance of dodging hits.
- The NecroShield suit allows you to take 1 hit without taking any damage, but takes 80 kills to renew.
- The Emergency Shell clears projectiles and gives you a shield whenever it activates (which is by a hit or by a dodge), which greatly reduces the chances of a 2nd hit
For most runs, you should either use the Emergency Shell (if you have it), or the Ninja Garb (if you don’t). (and you would probably Jackbot to 50 for the dodge bonuses as well).
The Necro also used to be the go-to for Campaign Endless because it was one-shot-death after Room 100, but since the endless rebalance it may be better to stay with the Ninja or Emergency Shell.
To get the best benefit of the Emergency Shell, you should learn to strategically take a little damage to activate the shell - for example the falling blocks in World 12 (only 250 damage), lava/spikes/mines in castles (only 1,000 damage) and the mines/pigs in titan runs.