TL;DR:
Global enhancements that
aren't PvP/Purple/ATO provide the most benefit by being the
lowest level possible.
I cut down this table from the
Rebirth Wiki to list only those which meet the criteria. Happy building.
Also these i24 IOs (the Wiki isn't up to date):
Unbreakable Guard: +7.5% Max HP (Level 20 to 50)
Reactive Defenses: Scaling Damage Resistance (Level 20 to 50)(Preventive Medicine: Absorb Proc is a proc with an auto-trigger, not a global, so any level works the same.)
Longer explanation:I like to fully optimize/min/max my builds. I typically create one build, then keep a fully "Boosted" (level 50) and a fully "Attuned" (all other levels) version at a trainer.
Currently, attuning (catalyzing) of enhancements on Rebirth seems to be restricted to enhancements with the attuned border.
Attuning enhancements is ideal for when you exemplar because your enhancements will function regardless of your character's level.
This would be especially ideal for global enhancements because they provide a boost to important stats on your character, which don't change regardless of your current level.
Boosting enhancements is ideal for level 50+ but provides no benefit to global enhancements (ignoring any combined power boosting stat, such as Luck of the Gambler: Defense/Increased Global Recharge Speed).
PvP IOs, purple IOs and ATOs (archetype origin enhancements,
excluding Winter's Gift) work at all levels as if they are attuned, so it's best to
boost them.
All other enhancements stop working when your character is more than 3 levels below the enhancement's level (except procs which work as long as the power is available).
So for situations where you are exemplar'd, you will typically want the
lowest level versions of global enhancements in order to give them the widest range of usability.
Thanks to MuonNeutrino for information.
