Synnar, as far as I know, was never said to be the embodiment of any concept or anything of the sort, and so shouldn't be considered an abstract even at his peak.
The Source? Also not sure what concept it embodies.
Ditto with any god-like characters (e.g Wally, Elaine, etc.), and most nigh-omnipotents (e.g. Imps)
Here's my list of the top of my head:
Lucifer: Embodiment of God's infinite will
Michael: Embodiment of God's infinite power
Spectre w/ Logoz: Embodiment of God's wrath
+ 2 Endless (Death? Dream? Destruction? Probably not Destiny, he doesn't really do anything)
^^ ...
Aside from the LT ... the others aren't abstracts good friend.
Protege is not human, but humanoid.
Scathan is an energy encased entity.
HOTI is "God's in-universe" power source. (sort of abstract but not exactly)
The Gems are physical objects, making the IG an artifact.
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The most powerful Marvel abstracts are supposed to be:
The Living Tribunal
The BeyonderS
Abraxas
Eternity/Infinity/Death/Oblivion/Entropy
Lord Chaos/Master Order
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The funny thing is, that topping them all ... (even the LT)
are Two Concepts that are literally in cahoots with TOAA.
Eulogy and Expediency ... of the Seven Friendless,
which are part of Eternity/Infinity's family.
These Two Concepts use 4th wall power to do their thing. (OAA style)
Anyway, if this includes these Two legitimate canon Marvel abstracts,
I don't know who can counter them, aside from the Morrison avatar and such. 🙂