Based on the feats that Freed showed me, I agree with
@Selly that he should be a S-Class Mage. Tbh, I'd say he should be S-Class and Natsu shouldn't.
Freed's magic isn't only powerful as hell, it's also quite hax. Remember the Battle of Fairy Tail arc when he tortured Elfman? Yeah, he was about to use a spell to kill him. First of all, his Dark Écriture: Pain gives him the ability to be on par with Kyôka since they both use torture magic. But this is just one of Freed's spells.
Dark Écriture: Absolute Shadow give Freed feats that bring him above Elfman and close to Mirajane's Satan Soul power level. Rustyrose was without a doubt the strongest opponent close after Azuma. In the end, he was quite easily overpowered by Freed and Bickslow, while Erza needed ultra PoF to defeat Azuma. Freed could solo Rustyrose if he didn't cast Tower of Dingir. And we all know that Rustyrose is more or less equal with Rufus.
We also have Jutsu Shiki, which is the best defensive spell that anyone from the Fairy Tail guild uses. We've seen that he can cover whole Magnolia with this spell, so the AoE is really high. Plus, he could use this for a large amount of time.
If we compare Freed's feats to other S-Class Mages:
Attack: Laxus > Mirajane > Mystogan > Erza =
Freed
Defense:
Freed > Erza > Mystogan > Laxus = Mirajane
Stamina: Laxus > Erza > Mirajane >
Freed = Mystogan
Intelligence:
Freed > Mystogan > Erza > Mirajane > Laxus
Speed: Laxus > Erza > Mirajane >
Freed > Mystogan
Spell diversity: Mystogan >
Freed > Mirajane > Erza > Laxus
AoE: Laxus > Mirajane >
Freed > Mystogan > Erza
If someone disagrees, tell me why. No fanboying or fangirling please.
Freed is without a doubt S-Class level. I'd say he is close to Erza/Mystogan level.