General note: you want to slot stuff that gives character-wide bonuses
and isn't going give set bonuses in relatively ineffective powers.
Why? That frees up slots in more effective powers for ordinary
enhancements to boost those more effective powers. For example, an
Invulnerability tanker's Resist Physical Damage grants 10%
smashing/lethal resistance; Temporary Invulnerability, 30%. I want
the space in Temp Invulnerability for normal enhancements.

Stuff that most people want that comes in at level 7:
 Panacea proc in Health.
 Karma Knockback Protection (if you don't have a defensive set with
  it), usually in Combat Jumping / Hover.
 Bonesnap Acc/Damage in Brawl.
 Rectified Reticle Increased Perception which stops those darn Night
  Widows blinding you all the time - if you have a power that takes To-Hit.
At level 10:
 Archetype Origin sets.
 Overwhelming Force (expensive, but a way to get a third set in early,
  and six-slotting it gives knockback protection).
At level 12:
 Celerity Run / Stealth in Sprint.
At level 17:
 Preventative Medicine Absorb Proc in Health (or maybe in a heal you
 plan to six slot).
 Performance Shifter proc and endmod as a pair in Stamina.
At level 18:
 Power Transfer proc and endmod as a pair in Stamina.

Travel power: Blessing of the Zephyr (level 7)
Melee: Touch of Death (22), Mako's Bite (27) - just replace
Ranged: Decimation (22), Devastation (27)
        Decimation has the unique Chance for Build Up, so you _can't_
        have more than one Decimation six-slot, but it's a good thing
        to have one
Melee AoE Damage: Multi-Strike (17, cheap)
                  Scirocco's Dervish (17, set bonuses good for endurance)
                  Obliteration (27, set bonuses good for recharge)
Target AoE Damage: Annihilation (17, good for end redux)
                   Positron's Blast (17, good for recharge)
                   Bombardment (27, good for range)
Healing: Preventative Medicine (17, for the Absorb Proc)
         Doctored Wounds (27, recharge boost 5-slotted)
These lists of uniques for Resist Damage and Defence are partly an
 artefact of the order I did stuff in when first back, but also a
 defensive power rarely wants more than 4 slots just to enhance itself.
Resist Damage: Titanium Coating (22, good if you only want a few)
               Aegis (22, 4+ slots, has the useful Psi/Status unique)
               Some uniques to go with Titanium Coating:
               Unbreakable Guard +7.5% HP (17)
               Steadfast Protection Resistance/Defence (7)
               Gladiator's Armour +Def(All) (7) - expensive
               Impervious Skin Status Resistance/Regeneration (7)
Defence: Luck of the Gambler (22) has a Global Recharge Reduction piece
         Cheaper is Gift of the Ancients (12)
         Unique: Reactive Defences Scaling Damage Resistance (17)
Hold: Essence of Curare (17), Lockdown (27) - just replace
To-Hit Buff: Adjusted Targeting (7)
             Gaussian's (17) has the handy Chance for Build Up.
Accurate Defence Debuff: Analyse Weakness (27)
To-Hit Debuff: Dark Watcher's Despair (17)
Recharge Intensive Pets: Expedient Reinforcement (30)
Pets that don't want Recharge Intensive: Sovereign Right (12)
Slow: Pacing of the Turtle (17)
Stun: Rope-a-dope (17)
Taunt: Mocking Beratement (17)
Immobilise: Trap of the Hunter (17)
Sniper: Executioner's Contract (17), Extreme Measures (35)
        Or consider just slotting it as a regular ranged attack.
Accurate To-Hit Debuff: Siphon Insight (27)

What to do at 50:
You can use Catalysts to upgrade archetype origin enhancements to
Superior versions of themselves. You probably want to do a whole set
in one go because the Superior version and the regular don't match for
set bonus purposes.

You can slot expensive "purple" sets, which have higher bonuses. These
should not be slotted attuned; they exemplar down fine anyway and the
unattuned ones can be "boosted" (with enhancement boosters) to 50+1,
+2... to +5, an expensive way to get very small gains.

Purple sets are all unique. The ones that boost attacks are worth it
if you are space rich enough to afford them; the others less so,
except maybe the pet set Soulbound Allegiance. In particular while
Unbreakable Constraint is stronger in other respects than other Hold
sets, it lacks the "chance for +2 magnitude" in Lockdown which can let
you hold a boss with a single cast.